Leading Republicans united Tuesday in an extraordinary denunciation of Donald Trump's attacks on a federal judge, with House Speaker Paul Ryan calling them the "textbook definition of a racist comment," though he stood by his endorsement of the presumptive presidential nominee.

Trump asserted that his comments were being "misconstrued," but did not back down or apologize for saying repeatedly that U.S. District Judge Gonzalo Curiel could not preside fairly over a case involving Trump University because of his Mexican heritage.

"I do not feel that one's heritage makes them incapable of being impartial, but, based on the rulings that I have received in the Trump University civil case, I feel justified in questioning whether I am receiving a fair trial," Trump said in a lengthy statement that repeated his claims that students at Trump University, far from being fleeced as some claim and as evidence suggests, were overwhelmingly satisfied.

Moments before Trump issued his defiant statement, a Republican senator who had previously indicated support for Trump withdrew his backing, as Republicans' attempts to unite behind Trump looked at risk of unravelling.

"While I oppose the Democratic nominee, Donald Trump's latest statements, in context with past attacks on Hispanics, women and the disabled like me, make it certain that I cannot and will not support my party's nominee for president regardless of the political impact on my candidacy or the Republican Party," Senator Mark Kirk of Illinois, who is in a competitive re-election race, said in a statement.

Donald Trump has contended that U.S. District Judge Gonzalo Curiel, who is presiding over a case alleging that Trump University fleeced students, can't judge him fairly because the judge is of Mexican heritage. (Richard Shiro/Associated Press)

"I have concluded that Donald Trump has not demonstrated the temperament necessary to assume the greatest office in the world," Kirk said.

Kirk was the first leading Republican to publicly disavow earlier support for Trump. Most others, including Ryan, reaffirmed their plans to support him, but the situation exposed the peril for Republicans with the volatile and unpredictable Trump as their standard-bearer. Time and again, they are forced to answer for Trump's latest divisive comment, distracting from their own agendas as well as their goals of winning back the White House and hanging onto Senate control.

On Tuesday, Republicans were squirming over what might have been the billionaire's most incendiary stance to date — the claim that Curiel couldn't preside fairly over the Trump University case because the U.S.-born judge is of Mexican heritage and Trump wants to build a wall between the U.S. and Mexico.

"I regret those comments he made. Claiming a person can't do their job because of their race is sort of like the textbook definition of a racist comment," Ryan said at a morning news conference where his attempts to focus on a new House Republican poverty-fighting agenda were overwhelmed by questions about Trump. "I think that should be absolutely disavowed. It's absolutely unacceptable."

Republican presidential candidate Donald Trump has said Curiel, shown here, can't judge him fairly because he is of Mexican heritage. (United States District Court, Southern District of California)

"But do I believe Hillary Clinton is the answer? No, I do not," Ryan said.

Others avoided the word racist but made their disapproval crystal clear.

"My advice to our nominee would be to start talking about the issues the American people care about and to start doing it now," said Senate majority leader Mitch McConnell. "In addition to that it's time, to quit attacking various people that you competed with, or various minority groups in the country and get on message."

Ron Weiser, one of the recently named top fundraisers for Trump and the Republican Party, said the nominee's comments on the judge are "obviously making it more difficult" to raise money.

'Out of line'

Stanley Hubbard, a Minnesota broadcast company billionaire, recently gave $100,000 to a pro-Trump group and describes himself as a reluctant Trump backer. He said of Trump's judge comments: "It's ridiculous. He's out of line. You don't attack a federal judge, and you certainly don't attack him on the heritage of his parents. It's totally off the wall, and I don't even have words to explain it."

Only his fear of Democrat Hillary Clinton picking Supreme Court justices is enough to keep him giving money to Trump, Hubbard said.

Senator Tim Scot, the only black Republican senator, called Trump's comments on the judge "racially toxic," yet said, "He needs to get on to the general election and we need to win."

"Let's face it, meet the old Trump, just like the new Trump," said Senator Jeff Flake, who has long opposed the billionaire's candidacy. "We've got what we've got. That's not somebody who can win the White House."

"Where there's no talk of a convention challenge or anything else, this might spur it," Flake added of Trump's comments on the judge.

Democrats ridiculed Republicans for denouncing Trump's comments yet continuing to back the mogul, in evidence of how much ammunition Trump is giving them as they try to boost their own deeply flawed presumptive nominee in Clinton.

"If Republicans believe that a man who believes in religious and ethnic tests for federal judges is fit to be president of the United States, they must explain why this is an acceptable position," said Senate minority leader Harry Reid.