Mr. Bloomberg also assailed Mr. Kennedy for discouraging measles vaccination during an outbreak in the island nation of Samoa, where 83 people died.

“Parents who have been swayed by vaccine skepticism love their children and want to protect them, and we need leaders who will help them do that,” he said, “not conspiracy theorists who will scare them into decisions that will put their children at risk of disease.”

Mr. Bloomberg has spent billions of dollars promoting public health, both through his charity, Bloomberg Philanthropies, and through donations to the School of Public Health at Johns Hopkins University, which now bears his name. The school and the charity hosted the health summit, with the theme of “advancing public health in uncertain political times.”

Like Mr. Kennedy, Mr. Bloomberg has fought battles against processed foods and has tried to promote healthy eating. But that, it appears, is where their like-mindedness ends.

Among other things, Mr. Bloomberg chided Mr. Kennedy for “nutty conspiracy theories,” including making the “outrageous false claim” that the Covid-19 shot was the “deadliest vaccine ever made.” He said Mr. Trump deserved credit for Operation Warp Speed, the fast-track initiative that produced coronavirus vaccines in record time, noting that studies have shown that the vaccines have saved an estimated 20 million lives around the world.

With experts warning of a possible bird flu outbreak in humans, Mr. Bloomberg said senators would face some hard questions: “With the nation facing a possible bird flu outbreak, are they really prepared to roll the dice on the lives of their constituents, by placing someone in charge of public health who has made it clear that he will prevent the approval of lifesaving vaccines?”

“I am totally against the once great and powerful U.S. Steel being bought by a foreign company, in this case Nippon Steel of Japan,” Mr. Trump wrote in a post on Truth Social. “As President, I will block this deal from happening. Buyer Beware!!!”

President Biden and Vice President Kamala Harris had also indicated this year that they opposed Nippon’s $15 billion bid for U.S. Steel, and the White House had appeared poised to block the transaction in September ahead of the election. Amid concerns that the review process was being politicized, the Biden administration agreed to grant a request by Nippon to resubmit its filing with the agency running the review, which is the Committee on Foreign Investment in the U.S., or CFIUS (pronounced SIFF-ee-yuhs).

That decision gave the two steel makers an additional three months to convince the U.S. government that the transaction did not pose a threat to national security, amid concerns from both Democrats and Republicans. That period is about to expire later this month, forcing the Biden administration to grant the companies another extension or to make a decision about the fate of the deal.

Mr. Trump said on Monday that his economic plan would ensure that U.S. Steel remained strong on its own without the promised investments from Nippon Steel.

“Through a series of Tax Incentives and Tariffs, we will make U.S. Steel Strong and Great Again, and it will happen FAST!” Mr. Trump said.

U.S. Steel was founded in 1901, and its metal has been used to build critical infrastructure across the United States.

The company’s fortunes have fluctuated with changes in global steel markets over the decades. In recent years its finances have become precarious, making it ripe for a takeover. U.S. Steel executives have argued that the deal with Nippon is necessary for the company’s future and that they will have to cut jobs otherwise.

The timing of the acquisition became politically fraught this year because U.S. Steel is based in Pennsylvania, which was one of the most crucial swing states in the election.

Some leaders of the powerful steelworkers union opposed the deal, fearing that Nippon would eventually focus its investments on facilities outside the state and that it would not honor pension agreements.

“The proposed sale is bad for workers, our communities and the domestic industry — as well as our national security, critical infrastructure and domestic supply chains,” the United Steelworkers union said on Monday. “We must continue to resist it.”

Critics of the deal have argued that it could be blocked on national security grounds because the United States would be giving up control of part of its steel supply chain to a foreign company. Proponents of the transaction note that Japan is a close U.S. ally and say that it does not pose a national security threat.

Backers of the deal include Mike Pompeo, who served as secretary of state under Mr. Trump and has been acting as an adviser to Nippon. Wilbur Ross, who was Mr. Trump’s commerce secretary, has also publicly called for the deal to be approved.

Nippon Steel has been focused on influencing the outcome of the deal, operating under the assumption that the final decision would rest with Mr. Biden. If the deal were to close before Mr. Trump took office, it could complicate his efforts to unwind it.

Company officials had expected CFIUS to complete its investigation of the deal by Dec. 23, after which Mr. Biden would have 15 days to announce a decision.

Takahiro Mori, the Nippon executive overseeing the U.S. Steel deal, has spent recent weeks traveling across the United States, meeting with local leaders of the United Steelworkers. He also held discussions with Gov. Josh Shapiro of Pennsylvania.

Internally, the company felt the meetings were productive, and there was optimism that a decision in favor of the deal could come before Christmas.

The White House and U.S. Steel did not immediately respond to requests for comment. Nippon Steel declined to address Mr. Trump’s latest remarks but said in a statement that it is “determined to protect and grow U. S. Steel in a manner that reinforces American industry, domestic supply chain resiliency and U.S. national security.”

“It will be a very special day for all!” he wrote.

The trip has been in the works for several days, according to people briefed on the planning. Mr. Trump and Mr. Macron have had at least one phone conversation, according to one of the people.

President Biden is not expected to attend the reopening, but Dr. Jill Biden, the first lady, will be there, according to one of the people briefed.

Mr. Trump has rarely left Mar-a-Lago, his private club and home in Palm Beach, Fla., since he won a second term by defeating Vice President Kamala Harris, who replaced President Biden on the Democratic ticket.

The news of the trip was in some ways unsurprising. Mr. Trump loves ceremony and grandeur as it relates to construction sites, especially historic ones. And it marks his return to the world stage.

But it is also the latest chapter in what has been a fraught relationship with European allies — and with Mr. Macron in particular.

The French leader, who is facing domestic turbulence after a wave of anger from far-right and far-left forces, flattered Mr. Trump early in his first term as U.S. president. Mr. Macron invited Mr. Trump to attend the country’s Bastille Day celebration in Paris in 2017, and Mr. Trump went eagerly.

But the relationship soured in 2018, when Mr. Macron endorsed the idea of a true European military defense, one that could counter Russia and China but also the United States. Mr. Macron’s approach chafed against Mr. Trump’s nationalism, at a time when far-right populists generally aligned with Trump were ascending in France and elsewhere in Europe.

When the F.B.I. searched Mar-a-Lago for hidden classified documents in August 2022, some of the information federal agents took from the property related to Mr. Macron.

The Bidens, their lawyers and their defenders have leveled that accusation against the president’s own appointees at the Justice Department since the summer of 2023, when a plea deal that would have granted Hunter Biden broad immunity collapsed — leading to the two indictments, and the likelihood that he would face significant prison time.

After the pardon announcement, Eric Holder, who served as attorney general under President Obama and then-Vice President Joe Biden, wrote in a social media post that no U.S. attorney “would have charged this case given the underlying facts,” and that “Had his name been Joe Smith, the resolution would have been — fundamentally and more fairly — a declination” to prosecute.

U.S. prosecutors have broad discretion when deciding whether to bring cases and are governed by departmental guidelines. But such decisions are subject to a wide range of factors that determine whether they opt to charge, to seek an agreement or to drop the matter altogether, even when there is some evidence of criminality.

The prosecution of the younger Mr. Biden on gun charges was relatively rare. Few people fitting his profile — a first-time, nonviolent offender accused of lying on a federal firearms application, who never used the gun to commit a crime — get serious prison time for the offenses charged in the indictment, according to former and current officials. He held onto the gun, a Colt Cobra .38, for less than two weeks, five years ago.

When officials with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives reviewed Hunter Biden’s gun application several years ago, they believed the case most likely would have been dropped if the offender were a lesser-known person, because the gun had not been used in any crime and because Mr. Biden had taken steps to get and stay sober, according to a former law enforcement official familiar with the situation.

It is harder to say whether Mr. Biden’s tax case, stemming from a five-year investigation into his foreign business dealings and profligate spending, was outside of norms. Such cases are not uncommon — although Mr. Biden’s legal team argued that, since he had fully paid his back taxes and the relevant penalties, a trial was unnecessary.

Justice Department officials were incensed by the suggestion that the cases were prosecuted for any reason other than merit. The special counsel in the tax case, David C. Weiss, pushed back against President Biden’s claims in a court filing on Monday, saying there “never has been any evidence of vindictive or selective prosecution in this case.”

Mr. Weiss rejected the idea that the gun and tax cases were politically motivated or represented selective prosecution, noting that judges in both had emphatically rejected such “baseless claims.”

“I’m disappointed this was the decision that he landed on here,” Representative Jason Crow, Democrat of Colorado, said of President Biden in an interview. “He promised he would not do this. I think it will make it harder for us going forward when we talk about upholding democracy.”

Representative Marie Gluesenkamp Perez, Democrat of Washington, said the president’s move offered proof of a two-tiered justice system that treated the wealthy and politically powerful differently from everyday Americans.

“The President made the wrong decision,” she wrote on social media. “No family should be above the law.”

In his own post on social media, Senator Michael Bennet, Democrat of Colorado, said Mr. Biden’s “decision put personal interest ahead of duty and further erodes Americans’ faith that the justice system is fair and equal for all.”

In an interview, Mr. Bennet compared the pardon to Mr. Biden’s decision to wait until the final months of his re-election bid to drop out of the race, calling it an example of the president’s “putting his personal interest ahead of his responsibility to the country.”

Many progressives have rallied behind Mr. Biden.

“Way to go Joe!” Representative Jasmine Crockett, a first-term Democrat from Texas, said on MSNBC over the weekend. “Let me be the first to congratulate the president for deciding to do this, because at the end of the day, we know that we have a 34-count convicted felon about to walk into the White House.”

But critics of Mr. Biden’s decision were not confined to the party’s moderate wing.

“President Biden’s pardon of his son Hunter is, as the action of a loving father, understandable — but as the action of our nation’s chief executive, unwise,” Senator Peter Welch, Democrat of Vermont and a progressive, said in a statement.

And Senator Gary Peters of Michigan, who headed the Democrats’ Senate campaign arm, called Mr. Biden’s decision to pardon his son “wrong.”

“A president’s family and allies shouldn’t get special treatment,” Mr. Peters said in a statement. “This was an improper use of power, it erodes trust in our government, and it emboldens others to bend justice to suit their interests.”

But Senator Richard J. Durbin of Illinois, the No. 2 Democrat in the chamber and a longtime colleague of the president’s, was among Mr. Biden’s defenders on Monday. Mr. Durbin said he understood the president’s “humane” decision, saying that Hunter Biden had been “exploited” for political purposes.

“Joe Biden is many things, but I can say for certain he is a loving father,” Mr. Durbin said in an interview. The president, he added, “would gladly go into that jail cell himself to spare his son that experience.”

Peter Baker and Carl Hulse contributed reporting.

About 250 hostages were captured in the Hamas-led attack on Israel on Oct, 7, 2023, and about 100 of them remain in Gaza, with about a third believed to be dead, according to the Israeli authorities. The Biden administration has been working since last year with Israel and international mediators, including Qatar and Egypt, on a cease-fire deal that would include release of the hostages.

It was not clear what tactic Mr. Trump might take that has not already been taken already by Israel, which has killed many of Hamas’s leaders and thousands of its fighters, while leveling much of Gaza.

Mr. Trump said in his post on Monday, “It’s all talk and no action” when it comes to the hostages, and his statement suggesting that he could use the power of his office to punish those who took the captives is the first sign of how aggressively Mr. Trump might handle Middle East policy when he resumes office.

There was one brief pause in the fighting, more than a year ago, that led to the release of about 105 hostages. And efforts to broker a cease-fire deal appeared to stall last month after mediators met in Egypt.

Some hostages have also been rescued in Israeli military operations. But there have been protests and widespread frustration in Israel over the lack of a deal to secure their freedom, with critics of Prime Minister Benjamin Netanyahu of Israel saying that he has failed to prioritize their release over his political survival.

This is not the first time that Mr. Trump has addressed the conflict in Gaza and the fate of the remaining hostages. He has told Mr. Netanyahu that he wants the war in the enclave to end before he returns to the White House. And at the Republican National Convention in July, he said that he wanted the hostages returned and that captors “will be paying a very big price.”

His latest warning came after Hamas released a propaganda video showing the American-Israeli hostage Edan Alexander, 20, pleading with Mr. Trump to secure his release.