Former Republican Sen. Jeff Flake on Monday, upon endorsing Democratic presidential nominee Joe Biden, blasted President Trump as an “embarrassment,” while slamming his claims that Democrats are trying to rig the 2020 presidential election.

Flake’s comments come after he and more than a dozen former Republican members of Congress threw their support behind a “Republicans for Biden” effort being launched Monday by Biden’s campaign to engage potential GOP supporters this November.

EXCLUSIVE: JEFF FLAKE JOINS OVER TWO DOZEN FORMER REPUBLICAN MEMBERS OF CONGRESS IN ENDORSING JOE BIDEN

Flake, who told reporters Monday that he’s never voted for a Democratic presidential candidate before, slammed the president and criticized him and his administration for being “assuredly not conservative.”

“Now, as the president of the United States, he has said, and I quote, the only way we're going to lose this election is if the election is rigged,” Flake said Monday. “What kind of president talks like that? What kind of American leader undermines confidence in the elections in his own country?”

He added: “It's part of his strategy to hold power. This is extraordinarily dangerous to a free society, and it stands to inflict lasting damage to our democracy.”

Flake went on to call Trump an “embarrassment to the office” of the presidency, and questioned whether anyone wants American children to “emulate” his behavior.”

“The president’s behavior has not and will not change,” Flake said. “And whatever else you call the behavior I have just described, it is not conservative.”

Flake went on to say Trump has a “disregard” for the separation of powers, and said “governing by tweet is not conservative.”

“It’s not even governing,” he said.

Flake said Republicans cannot claim that the party has “remained faithful to conservative principles” during the Trump administration.

“There's less of a conservative case to be made for re-electing the president,” Flake said.

Flake touted Biden as the “kind of Democrat” who he is “confident will approach the constitutional role with the reverence and dignity it deserves.”

“I know that he will reach across the aisle, because that’s what he’s done his entire career,” Flake said, calling Biden a “good and decent man.”

“I haven’t always agreed with him, and there will be many policies on which we will disagree in the future,” Flake said. “And that’s okay.”

He added: “The steadiness of leadership and the health and survival of our democracy, those things far supersede any policy issues on which we might agree or disagree.”

Flake said it is “because of his conservatism” and his “belief in the constitution and in the separation of power,” as well as his concern about the “conduct and behavior of our current president that I stand here today proudly and wholeheartedly to endorse Joe Biden as the next president of the United States of America.”

Biden's list of Republican supporters, shared first with Fox News, includes a number of well-known Trump critics, most notably Flake.

Last fall, he penned an op-ed urging lawmakers to abandon the president and save their "souls," as he backed impeachment. He's since said he won't vote for Trump, but had held off on a formal Biden endorsement until now.

Some others on the list had already backed the former vice president, including former Republican Sens. Gordon Humphrey of New Hampshire (who is now an independent) and John Warner of Virginia. They're joined by a number of former Republican House members:

Former Reps. Steve Bartlett of Texas, Bill Clinger of Pennsylvania, Tom Coleman of Missouri, Charlie Dent of Pennsylvania, Charles Djou of Hawaii, Mickey Edwards of Oklahoma, Wayne Gilchrest of Maryland, Jim Greenwood of Pennsylvania, Bob Inglis of South Carolina, Jim Kolbe of Arizona, Steve Kuykendall of California, Ray LaHood of Illinois (who served as Transportation secretary in the Obama administration), Jim Leach of Iowa, Connie Morella of Maryland, Mike Parker of Mississippi, Jack Quinn of New York, Claudine Schneider of Rhode Island, Chris Shays of Connecticut, Peter Smith of Vermont, Alan Steelman of Texas, Bill Whitehurst of Virginia, Dick Zimmer of New Jersey, and Jim Walsh of New York.

TRUMP ACCUSES DEMOCRATS OF 'USING COVID TO STEAL OUR ELECTION'

A Biden campaign official told Fox News the endorsements are a “strong rebuke” of Trump and his administration.

The “Republicans for Biden” program is part of the campaign’s national effort to engage Republicans who plan to vote for the Democratic nominee in November.

The Biden campaign is urging Republicans who plan to support Biden in the general election to encourage other Republicans to organize in their communities by using their “Vote Joe” app and other tools. A Biden campaign official said the app leverages existing relationships among supporters and allows them to identify and engage new supporters and potential persuadable supporters.

The campaign also sends mass text messages from the app encouraging volunteers and providing updates on events.

Trump campaign communications director Tim Murtaugh dismissed the latest announcement in a statement to Fox News: “Joe Biden has been a failure in the Washington Swamp for a half century, so no one should be surprised when Swamp creatures gather to protect one of their own.

"President Trump has unprecedented support — over 95 percent — among real Republican voters," the statement continued, "and is also making strong inroads in Biden’s core Democrat constituencies, like Black Americans, Latinos, and union members. President Trump’s record of success for all Americans will carry him to victory in November.”

RNC spokesman Steve Guest said Flake “has abandoned any set of principles he once professed to have in order to embrace Joe Biden, a far-left Democrat, who is the bannerman for AOC & Bernie Sanders’ $93 trillion Green New Deal agenda of raising taxes on the middle class and forcing government control of health care on to the American people.”

Fox News' Madeline Rivera contributed to this report.