What started as a trickle of "Never Trumpers" has turned into a historic wave of defections from high-profile Republicans.
By the close of the 2020 Republican National Convention, nearly 500 current and former GOP officials have gone public opposing a second term for the president of their own party.
"Absolutely unprecedented; nothing remotely like it," said presidential historian Mark Updegrove.
The divide over President Donald Trump and Trumpism has raised existential questions for Republicans both about party identity and loyalty.
"It's become the party of Donald Trump and any whim he has," Updegrove said. "It's about personality and not political party or platform."
While nine in 10 Republican voters approve of Trump as president, opposition to his leadership inside the party establishment has mushroomed.
The protests span the ideological and generational spectrum on the political right -- from former Ohio Gov. John Kasich and one-time GOP presidential nominee Sen. Mitt Romney to former Trump national security adviser John Bolton and Defense Secretary James Mattis.
"He is going to hear from more people who served in his administration and hear more of them give the same testimony I gave, which is that he's ill-equipped to hold the office that he has," 33-year-old former Trump Homeland Security chief of staff Miles Taylor told ABC's "Good Morning America" last week.
Already ex-Trump communications director Anthony Scaramucci and former Navy Secretary Richard Spencer have been loudly warning against the reelection of their former boss.
"Internal party conflicts have certainly happened before," said Princeton University political historian Kevin Kruse, "but we’ve never really seen anything of this size and scope. In all, the avalanche of criticism from Republican officials, past and present, against a sitting Republican president is stunning."
A group of 73 veteran Republican national security officials is running newspaper ads calling Trump a "danger." The GOP activist group The Lincoln Project is spending millions attacking the president in TV ads airing on Fox News.
On Monday, former Arizona Sen. Jeff Flake, who's never voted for a Democrat, joined more than two dozen Republican former members of Congress to publicly endorse Joe Biden.
"It is because of my conservatism and because of my belief in the Constitution and in the separation of powers, and because I'm gravely concerned about the conduct and behavior of our current president," Flake said.
Four Republicans last week -- including former Secretary of State Colin Powell -- went even further, speaking out against the president at the Democrats' convention in prime time.
"An avalanche like this -- and not just of low-level aides -- has certainly never happened in modern history," said political historian Allan Lichtman of American University. "It's a sign that at least at the ideological level, that this is a major rift within the Republican Party."
It's an extraordinary political family feud that many in the party's rank-and-file seem to be shrugging off.
"Donald Trump is going to win in November, and the reason he's going to win is because of the results of the last four years," said Republican congressional candidate Jake LaTurner of Kansas.
The president retains sky-high approval ratings from his base, drawing enthusiastic crowds and raising record-setting cash -- more than $165 million in July alone.
National party leaders insist the values of Republicanism remain unchanged, and that intraparty rebellions and high-profile defections are nothing new.
In 2004, Democratic Sen. Zell Miller of Georgia made waves when he delivered the RNC keynote address endorsing George W. Bush., and in 2008, Democratic Sen. Joe Lieberman crossed the aisle to endorse his friend Republican John McCain.
"Endorsements don't matter much," said Lichtman. "For thoughtful persons they may be significant, but there's not a correlation historically between endorsements and election results."
But never have aisle-crossing endorsements reached this scale or had such vigor, giving some of the president's most loyal supporters reason for concern.
Evangelical pastor Bart Barber of Farmersville, Texas, a loyal Republican who plans to vote for Trump in November, told ABC News he's worried about what impact the president's behavior will have on the future of the party.
"I wouldn't hold up the president as an example of the kind of moral or religious perspective that I preach and that our church believes in and represents," Barber said. "I absolutely think the Republican Party has lost ground morally."
There's also tension over Trump's embrace of the political fringe, from birtherism and white nationalism to baseless online conspiracy theories like QAnon, which the FBI calls a domestic terror threat.
"The president often talks about how he gets a lot of ratings, but at the end of the day, people want problems solved -- not ratings or personal popularity," said former Virginia GOP congresswoman Barbara Comstock. "That's why some of our Republican governors are the most loyal people, whether or not they are loyal to this president."
Whether or not Trump wins in November, many in the party are resigned to the belief that Trumpism will be part of Republicanism for the foreseeable future.
"There's no question: this is Donald Trump's Republican party. But politics is often an exercise in addition, and certainly winning a campaign requires you to reach out to other people," said Sara Fagen, a former political adviser to President George W. Bush and ABC News contributor.
"We're in a place now where Donald Trump is behind," Fagen said, "and there's about 12% of the electorate who are soft Republicans. They like Donald Trump's policies, but they don't like him. But guess who they do like? They like President Bush; they like Mitt Romney. And (Trump is) going to have to appeal to that piece of Republican Party."
ABC News' Janet Weinstein contributed reporting.