Retired federal appeals court Judge J. Michael Luttig, a prominent conservative legal scholar put on the bench by President George H.W. Bush, is endorsing Vice President Kamala Harris over former President Donald Trump, whose candidacy he describes as an existential threat to American democracy.

It will be the first time Luttig, a veteran of two Republican administrations, has voted for a Democrat.

“In the presidential election of 2024 there is only one political party and one candidate for the presidency that can claim the mantle of defender and protector of America’s Democracy, the Constitution, and the Rule of Law,” Luttig wrote in a statement obtained exclusively by CNN. “As a result, I will unhesitatingly vote for the Democratic Party’s candidate for the Presidency of the United States, Vice President of the United States, Kamala Harris.”

Luttig played a now famous role in persuading then-Vice President Mike Pence to defy Trump and certify the 2020 presidential election. In a series of tweets drafted at the request of Pence’s attorney, Luttig spelled out in stark terms the legal rationale for Pence to reject the former president’s attempt to overturn Joe Biden’s victory.

Since then, Luttig has emerged as a preeminent constitutional critic of Trump. In endorsing Harris, Luttig argues that partisan distinctions must, in this election, be set aside in order to prevent the “singularly unfit” Trump from returning to the White House.

“In voting for Vice President Harris, I assume that her public policy views are vastly different from my own,” Luttig writes, “but I am indifferent in this election as to her policy views on any issues other than America’s Democracy, the Constitution, and the Rule of Law, as I believe all Americans should be.”

Luttig’s scathing rebuke of Trump and endorsement of Harris underscores the depths of divisions between Reagan-and Bush-era Republicans and the modern, Trump-dominated GOP. The former judge is just as unsparing a critic of the Republican party as he is of Trump, whom together he says have launched “the war on America’s Democracy.”

The corrosive effects, he adds, will echo through generations.

“Because of the former president’s continued, knowingly false claims that he won the 2020 election, millions of Americans no longer have faith and confidence in our national elections, and many never will again,” Luttig writes.  “Many Americans – especially young Americans, tragically – have even begun to question whether constitutional democracy is the best form of self-government for America.”

The stakes, Luttig argues, are as high now as in the late 18th century, when the country’s founders and authors of the US Constitution – including Alexander Hamilton and Thomas Jefferson, typically political foes – joined together to voice concern over the potential emergence of an authoritarian demagogue.

“The time for America’s choosing has come,” Luttig writes. “It is time for all Americans to stand and affirm whether they believe in American Democracy, the Constitution, and the Rule of Law, and want for America the same – or whether they do not.”

Though this will be Luttig’s first time pulling the lever for a Democrat in any election, he has, in the aftermath of January 6, 2021, come out in support of some decisions by the Biden administration. He wholeheartedly endorsed the 2022 nomination of now-Supreme Court Justice Ketanji Brown Jackson to the high court, even calling out Republicans who said they would not vote to confirm her.

“The President knew at the time that there were any number of highly qualified black women on the lower federal courts from among whom he could choose – including Judge Jackson – and Republicans should have known that the President would nominate one of those supremely qualified black women to succeed Justice Breyer,” he wrote at the time.

Luttig now joins a number of high-profile Republicans endorsing Harris, including former members of Congress Joe Walsh, Barbara Comstock and Adam Kinzinger.

Kinzinger, now a CNN contributor, will have a high-profile speaking slot this week at the Democratic National Convention in Chicago.

Former Georgia Lt. Gov. Geoff Duncan, also a CNN contributor, endorsed Harris at the end of July in an Atlanta Journal-Constitution op-ed.

Her campaign, he wrote, was “the best vehicle toward preventing another stained Trump presidency.”

Speaking to CNN, Luttig said his decision to publicly back Harris was a matter of knowing “right from wrong” – and acting in accordance.

“In my faith, we believe that we will one day answer for our wrongs. I have always tried to live my life in anticipation of that day. Imperfectly, to be sure.  But I have tried,” an emotional Luttig said. “My endorsement of the Vice President was the right thing to do.  It would have been wrong for me to stay silent, and I believe I would have one day had to answer for that silence.

“It’s really that simple.”