ATLANTA — In a presidential election rematch where every vote may count, the campaigns of President Biden and former President Trump are reaching out to key constituencies in an attempt to gain an advantage in what may end up being a photo finish.

Aiming to court the small but potentially crucial sliver of moderate Republicans who are disgruntled with Trump as their party's standard-bearer, Biden's campaign on the eve of the first presidential debate unveiled an endorsement from former GOP Rep. Adam Kinzinger of Illinois.

The former military pilot and Iraq War veteran who transformed into a major Republican Trump critic after the Jan. 6, 2021, attack on the U.S. Capitol by Trump supporters aiming to upend congressional certification of Biden's 2020 election victory on Wednesday charged that the former president is "a direct threat to every fundamental American value" in a video announcing his endorsement.

Hours later, standing in the Georgia State Capitol in Atlanta, Kinzinger said that "if you’d have told me three years ago that ‘You’re going to be endorsing a Democrat for president in three years,’ I probably wouldn’t have believed you."

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Former GOP Rep. Adam Kinzinger of Illinois, center, speaks at a Biden campaign news conference in Atlanta on Wednesday. Joining him, from left, are former Georgia Lt. Gov Geoff Duncan, former U.S. Capitol Police officer Harry Dunn and Georgia Senate Minority Leader Gloria Butler. (Fox News/Paul Steinhauser)

"But I got to tell you, the stakes of this moment are way too high," Kinzinger added.

Another vocal GOP Trump critic, former Lt. Gov. Geoff Duncan, joined Kinzinger at the news conference, along with former U.S. Capitol Police officer Harry Dunn, who defended the Capitol on Jan. 6, 2021, and Georgia Senate Democrat Leader Gloria Butler.

"Going against the grain as a Republican and supporting Democrat Joe Biden for president is not easy, but I'm not looking at this election through the lens of being a Republican," Duncan explained. "I’m looking through the lens of being an American. An American that cares more about the future of my country than the [morally] bankrupt nominee of my party."

Kinzinger's endorsement and the news conference came on the same day that the Biden campaign launched a new ad in the key battleground states that targets Trump over his actions during the attack three and a half years ago on the U.S. Capitol.

The spot, shared first with Fox News on Wednesday, features Genesee County, Michigan, Sheriff Chris Swanson discussing how he watched in horror as Trump supporters stormed the Capitol and attacked Capitol police officers.

"As a sheriff, it’s awful to watch police officers getting attacked. That’s not supporting this badge and this uniform. I have no desire to work with somebody who divides. That’s not what America is," Swanson emphasized.

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The ad, the endorsements and the news conference were all part of the Biden campaign's push to hammer the presumptive GOP presidential nominee over what they call "Donald Trump's attack on American democracy."

Biden has long made what he charges is Trump's threats to democracy a key focus of his presidency, and he has hammered home the point as he runs for a second term in the White House. It is very likely Biden will spotlight the issue in Thursday's first presidential debate between the two major party standard-bearers.

It's a pitch that may help the Biden campaign persuade some of the Republicans who supported former ambassador and former South Carolina Gov. Nikki Haley in the GOP presidential primaries to consider voting for a Democrat this autumn.

Former Republican Rep. Adam Kinzinger of Illinois speaks with Fox News after headlining a Biden campaign news conference at the Georgia State Capitol in Atlanta on Wednesday. (Fox News/Paul Steinhauser)

"There is still a group of Republicans that are kind of very uneasy going with Donald Trump. They feel like… it’s unthinkable to vote for a Democrat. So, they’re trying to sit here and figure it out," Kinzinger said. "This is an important time for Republicans to come forward and say it’s OK. You don’t have to give up your Republican stripes to vote for a Democrat. All you’re standing for is democracy."

Asked by Fox News if the Biden campaign's efforts to attract Republicans are adequate, Kinzinger said, "I think the campaign has certainly made it very clear, particularly in the last few months, that they want to win those Haley voters, those swing Republican voters."

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Kinzinger becomes the highest-profile former GOP elected official to formally back Biden, whose campaign earlier in the month named Kinzinger’s former chief of staff, Austin Weatherford, to serve as its national Republican outreach director.

The Trump campaign took aim at Kinzinger, who was one of only two Republicans who served on the Democrat-dominated House committee that investigated the attack on the Capitol.

Trump campaign spokesperson Steven Cheung took to social media, asking, "Who’s Adam Kinzinger? Isn’t he the crybaby?"

Republican Reps. Wesley Hunt of Texas, left, and Byron Donalds of Florida, third from left, and former HUD Secretary Dr. Ben Carson, second from right, attend a Trump campaign Black business roundtable discussion in Atlanta on Wednesday. (Fox News/Matthew Reidy)

While the Biden campaign was holding their news conference at the Georgia State House, Trump's campaign was making a pitch for Black voters at an event just a couple miles away.

The Trump campaign organized what it called a Black American Business Leaders Barbershop Roundtable discussion in midtown Atlanta.

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Leading the discussion were two of Trump's most prominent Black allies and surrogates: GOP Reps. Byron Donalds of Florida and Wesley Hunt of Texas, and Dr. Ben Carson, a 2016 Republican presidential candidate who later served as Housing and Urban Development secretary in the Trump administration.

Both Carson and Donalds are widely considered to be on the larger list of potential Trump running mates.

Republican Reps. Byron Donalds of Florida, left, and Wesley Hunt of Texas headline a Black voter outreach event titled "Congress, Cognac, and Cigars" in Philadelphia on June 4. (Fox News/Paul Steinhauser)

Later on Wednesday, 24 hours ahead of the first presidential debate that is being held in Atlanta, Hunt and Donalds were set to hold their second edition of "Congress, Cognac, and Cigars," a GOP outreach effort for Black male voters that they're holding in some of the key swing states.

Polling this year indicates that while Biden retains a very large margin of support among Black voters, Trump has made gains with the key electorate that for generations has been a major part of the Democratic Party base.

Get the latest updates from the 2024 campaign trail, exclusive interviews and more at our Fox News Digital election hub.