WASHINGTON • Many say the populist crazy talk is typical of the White House primaries, but Republican front runner Donald Trump's increasingly incendiary remarks are leading some conservatives to brand him a "fascist" and party rivals to ramp up attacks against him.
Most spectacularly, the real estate tycoon recently said he would support registering Muslims in a database, and insisted - despite lacking any evidence - he saw Arabs in New Jersey cheer when the Twin Towers fell on Sept 11, 2001.
His stance has become so belligerent that voices are asking, even inside his party, whether he is committed to democratic values.
Republican experts are warning that Mr Trump could do lasting damage to the Republican Party, and that his nomination in the party primaries would essentially hand the presidency to Democratic front runner Hillary Clinton.
Several campaign teams in the primary race now appear to be coalescing around the need to oppose the celebrity billionaire's candidacy.
Establishment conservatives even took the unfathomable step of using the F-word against a member of their own party.
"Trump is a fascist. And that is not a term I use loosely or often. But he has earned it," Mr Max Boot, a military historian and foreign policy adviser to Republican presidential hopeful Marco Rubio, posted on Twitter.
Said Mr John Noonan, a national security adviser to presidential hopeful and former Florida governor Jeb Bush: "Forced federal registration of US citizens, based on religious identity, is fascism. Period."
In its Tuesday editorial, the New York Times said the past week of the campaign had been "dominated by Donald Trump's racist lies".
The Seattle Times used similarly strong language in a Wednesday editorial that denounced Mr Trump's "button-pushing lie after button-pushing lie".
Some campaigns have seemed reluctant to directly take on the Trump machine. But Mr Bush, struggling to gain traction in the race, piled on, telling Fox News on Wednesday that Mr Trump has been creating "an alternative universe" with his harsh rhetoric, particularly about Muslims.
"He doesn't know what he is talking about," Mr Bush said of Mr Trump's assertion that thousands of people were cheering on 9/11.
But Mr Trump's campaign has shown extraordinary resilience, and "the Donald" remains atop all major new polls despite fact-checkers debunking many of his statements. "They say that Trump can do almost anything, and nobody leaves me. And it is true," Mr Trump said at a rally on Tuesday in South Carolina.
Republican groups are reportedly preparing attack ads against him. A few anti-Trump videos have recently emerged. Ohio Governor John Kasich has launched a one-minute Web ad that links Mr Trump to Nazi Germany.