Vice President Kamala Harris and former President Barack Obama teamed up for their first joint rally Thursday to take swings at former President Donald Trump and implore Georgians to vote early as Harris kicked off her "When We Vote We Win" concert series at a stadium outside Atlanta.

"So much is on the line in this election, and this is not 2016 or 2020," Harris said. "The stakes are even higher, because over the last years, and in particular the last eight years, Donald Trump has become more confused, more unstable and more angry. You see it every day. He has become increasingly unhinged."

Harris referenced comments made by Trump's former chief of staff Gen. John Kelly, who gave interviews this week saying the former president once said he wanted generals like those that Adolf Hitler had. Trump told reporters in Las Vegas on Thursday he never made those comments.

Harris said voters had a choice about who to put in the Oval Office.

"It's either Donald Trump, in there stewing ... over his enemies list or me, working for you, checking off my to-do list," Harris said. "You have the power to make that decision."

In his remarks, Obama also referenced sharp criticism of Trump by former military generals -- such as Kelly and Gen. Mark Milley, the former chairman of the Joint Chiefs of Staff.

"They are not, quote-unquote woke liberals," Obama later said. "They are people who have never in the past even talked about politics because they believe that the military should be above politics. But the reason they're speaking up is because they have seen that, in Donald Trump's mind, the military does not exist to serve the Constitution or the American people."

Obama said of Trump, "He doesn't see being commander-in-chief as a solemn, sacred responsibility. Just like everything else, he thinks the military exists to do his bidding, to serve his interests."

Bruce Springsteen, whose music has peppered many Democratic presidential candidates' set lists, performed several of his hits at the concert, while stars with Georgia ties including Spike Lee, Samuel L. Jackson and Tyler Perry took the stage to express their support for Harris. The Harris campaign said it sees these large events as a way to draw in large crowds and get them to vote and volunteer.

The campaign said 23,000 people were in attendance. However, there appeared to be some logistical and technical challenges. At one point, hundreds of attendees chanted that they could not see the stage, asking for the camera broadcasting the event on large monitors to zoom in. When Harris took the stage, her campaign song "Freedom" by Beyonce did not play as it typically does. The vice president walked out to no music. Many attendees were also seen walking out during Harris' speech.

Harris is next set to appear with former first lady Michelle Obama in Michigan on Saturday, campaign officials said.

The Obamas endorsed Harris in July and both spoke at the Democratic National Convention in August.

Asked about her support from Obama and Springsteen while speaking to reporters in Philadelphia on Thursday, Harris said she's "honored" to have them at her campaign events.

"I'm very honored to have the support of former President Obama. As you know, he's been on the campaign trail and has been really wonderful and extraordinary in terms of the time and effort that he's putting into our campaign, and people like Bruce Springsteen, to have their support, and of course, he is an American icon," Harris said. "I think it just shows the breadth and depth of the support that we have and also the enthusiasm that a lot of people are bringing to the campaign and feel about our campaign."

As of Wednesday, nearly 2 million Georgians voted early, surpassing the similar first eight days of early voting in 2022, 2020 and 2018, according to the office of the Georgia Secretary of State.

Georgia is a prime target for the Harris campaign as they look to hold on to a state President Joe Biden won by only 11,779 votes. And it's bound to be close again. Former President Donald Trump is currently leading Harris in Georgia by 1.5%, according to 538's polling average.

In the final days of the race, Harris' team has enlisted celebrities to help share her message and get voters out during early voting periods across the country. On Saturday, Harris was joined by Lizzo in Detroit and Usher in Atlanta, while on Tuesday, Eminem introduced Obama at a Detroit rally.

Also, megastar Beyoncé is set to join the vice president at a rally in the singer's hometown of Houston on Friday.

ABC News' Sarah Beth Hensley contributed to this report.