When Vice President Kamala Harris took over as Democrats' presidential nominee, she shunted President Joe Biden's old messaging about the "soul of the nation," leaning instead into "joy" and the "opportunity economy" while painting her opponents as "weird."

Now, in the home stretch, Harris is reviving messaging about former President Donald Trump's supposed threat to American democracy, resuscitating a tactic that was unable to stop her boss' polling skid but that Democrats hope could be one part of a winning closing argument amid increasingly dark rhetoric from her challenger.

"I'm really happy they're bringing back the democracy argument because I think it does create a permission structure for people who are anti-Trump Republicans, and it does fire up our base. So that's a twofer," said veteran Democratic strategist Peter Giangreco. "You got to keep the abortion stuff going. You got to keep the economic contrast going, where he's going to give breaks to billionaires and she's going to cut taxes."

"It's not one size fits all," Giangreco added. But Trump "made a major mistake, I think, with 'the enemy within.' I think it's a giant opening, and I'm glad they're jumping on it."

The tactical shift comes as Trump intensifies talk of "the enemy within" amid concerns that Election Day will be peaceful.

"We have some very bad people. We have some sick people, radical left lunatics," Trump said Sunday on Fox News. "And I think it should be very easily handled by, if necessary, by the National Guard or, if really necessary, by the military, because they can't let that happen."

Harris has seized on the remarks, appearing more open to dubbing the former president a "fascist," and in her own interview on Fox News Wednesday said it is "clear to me" that Trump is "unfit to serve, that he is unstable, that he is dangerous."

"This is a democracy, and in a democracy, the president of the United States in the United States of America should be willing to be able to handle criticism without saying he'd lock people up for doing it," she said.

"If he wins, he'll ignore all checks that rein in a president's power," a narrator warned in a new Harris ad released Thursday.

The willingness to focus on the alleged threats Trump poses to the country's democratic foundation marks a reversal from Harris' initial strategy from the summer, when media coverage was awash with notes of how different her more joyful tone stood compared to Biden's graver campaign, which focused extensively on the nation's character.

That shift took place amid a broader Democratic scramble to change things up from Biden's failed campaign, which was kneecapped after his ruinous June debate performance. But Democrats who spoke to ABC News insisted that the democracy missive remains a potent one -- particularly given Trump's rhetoric -- and that Biden's inability to prosecute the case had more to do with the messenger than the message.

"It is imperative now on Kamala to make sure the American people understand what the choice is in this election," one Democratic pollster said.

"The challenge with Biden is that optically, and this is unfair and unfortunate, but people just could not get past the optics of a frail-looking elderly man trying to be president for another four years," the person added.

This time, with a different messenger, Harris' team is hopeful that mixing in warnings about democracy could appeal to a small sliver of voters who are truly undecided about who to vote for, but even more voters who are frustrated with their choices and mulling whether to vote at all -- particularly, Democrats said, Trump-skeptical, Harris-curious conservatives.

There's already precedent for such a strategy to break through.

Facing the prospect of a red wave in 2022, Democrats instead defied the odds by expanding their Senate majority and keeping House losses to a minimum on a playbook of discussing the Jan. 6, 2021, insurrection and the revocation of federal abortion protections. And research shared within the campaign showed that casting Trump as unstable and Harris as a steady leader serves as an effective way to persuade voters this cycle, according to a source familiar with the matter.

"There's a very small share that is ... debating between the vice president and Trump. But there is a bigger share of these persuadable voters who are trying to decide between a third-party or writing in or just staying home. And I do think that argument gets some real traction, especially with older voters and a lot of the voters who may be more conservative-leaning voters who maybe have misgivings about both candidates," said one source familiar with the Harris campaign's thinking.

Robert Blizzard, a GOP pollster, said Democrats could find repeated success with such an argument.

"I'm not sure if voters haven't made up their minds about that," he said when asked whether voters had already made up their minds about if "democracy" is a chief issue for them. "But leaning into the 2022 playbook that helped Democrats stem the tide of a red wave -- the democracy angle coupled with abortion, certainly, was effective as a contrast for the left."

Trump's campaign, for its part, dismissed the new tactic.

"It's the end of the fourth quarter and Kamala is making the same losing play that has never won Democrats the game. Nobody believes the lie that President Trump is a threat to democracy because he was already president for four years and our democracy, including our standing on the world stage, was stronger than it is today under Kamala Harris. Kamala is resorting to lies because she can't talk policy," spokesperson Karoline Leavitt said.

To be certain, Democrats are not predicting Harris completely replaces her messaging on the "opportunity economy" and other campaign planks for democracy, instead saying she'll likely have it as one of many tools in her toolbox.

"I do not believe preserving democracy is going to be their closing argument. I expect that they will go back to some version of turning the page, moving forward, working together, the opportunity economy, finding ways to secure the border," said Jim Kessler of the center-left Third Way think tank.

Still, "Trump, in unscripted moments, has given them opportunities. And part of that is democracy," Kessler added.

And, Democrats said, Harris would be remiss to not take advantage of any opening in a home stretch when any one issue could help get a voter off the sidelines. And with tight polling and such a short runway left in the election, the race has morphed into trench warfare, leaving both sides looking for any way to have the edge when the fighting ends on Nov. 5.

"I see it more as people who maybe are still not sure they feel like voting matters or that they want to vote, and this is a way to give them a stake in the election," said one Democratic strategist with ties to Harris' team.

"At this point in the campaign," Kessler added, "if they're generating stories that put Trump and his team on the defensive, then they're succeeding on that day."