“It is unfortunate that my comments have been misconstrued as a categorical attack against people of Mexican heritage,” the presumptive Republican nominee for president said in a carefully worded but lengthy statement.

“I do not feel that one‘s heritage makes them incapable of being impartial,” the 69-year-old billionaire backtracked.

Trump’s statement came hours after House Speaker Paul Ryan, the most senior elected Republican in the country who endorsed his candidacy only last week, described his criticism of the judge as “racist.”

“Claiming a person cannot do the job because of their race is sort of like the textbook definition of a racist comment,” Ryan told reporters. “I think that should be absolutely disavowed. It‘s absolutely unacceptable.”

Mark Kirk, a Republican senator from Illinois, broke ranks and said he was un-endorsing Trump, calling his remarks “dead wrong” and “un-American.”

“I cannot and will not support my party’s nominee for president,” Kirk said. “I have concluded that Donald Trump has not demonstrated the temperament necessary to assume the greatest office in the world.”

Trump inflamed the U.S. political establishment but emerged victorious in the Republican race to run for president with an controversial campaign denouncing illegal immigration, promising to build a wall along the southern U.S. border and branding Mexicans criminals, as well as insulting women and the media.

He asserted in an interview published last week that Curiel has “an absolute conflict” of interest because of his “Mexican heritage” and because he is a member of a Mexican-American lawyers association.

With Clinton poised to make history as the first woman nominee for president of a major U.S. party, Republicans rounded on Trump in public for questioning the bedrock of American democracy -- an impartial judiciary.

Despite his climb down, Trump insisted he was justified in asking whether he will receive a fair trial, currently slated to start weeks after the November general election.

Trump said “unfair and mistaken rulings,” the judge‘s “reported” associations and his status as an Obama appointee, meant it was fair to raise questions regarding his impartiality given his “unique circumstances as a nominee of the Republican party” and given the issues on which he has campaigned.

He lashed out at the media for reporting “one inaccuracy after another” in the Trump University litigation, insisted that the students were given a “valuable education” and claimed he would win the case.

Other top Republicans came forward to rebuke the businessman, whose campaign has been stuffed with insults against women, Muslims, the disabled, the press and replete with provocative foreign policy proclamations.

The tycoon’s former White House rival Jeb Bush fired back after his attempt at damage control, tweeting: “Donald Trump should retract his comments, not defend them. There is no place for racism in the GOP, or this country.”

Senate Majority Leader Mitch McConnell, whom Trump recently boasted of being able to rally round his candidacy, urged him to “get on message” immediately.

“My advice to our nominee would be to start talking about the issues that the American people care about and to start doing it now,” he told reporters.

“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.”

Even New Jersey Governor Chris Christie, who was the first high-profile Republican to endorse Trump -- while denying his “friend” was a racist -- suggested he needed to take stock.

“But whether he can learn anything or not, only time will tell about that.”

Christie followed Ryan’s lead in pivoting to attack Clinton -- accusing her of acting unjustifiably “high and mighty” towards her Republican rival.

“Do I believe that Hillary Clinton is the answer? No, I do not,” Ryan said.

“I believe that we have more common ground on the policy issues of the day, and we have more likelihood of getting our policies enacted with him than we do with her,” the speaker said of Trump. (AFP)