By Doina Chiacu

WASHINGTON (Reuters) - U.S. Republican presidential candidate Donald Trump on Monday rejected a barrage of criticism from his own party over his allegations of bias against a Hispanic judge, insisting his concerns were valid.

"All I want to do is figure out why I'm being treated unfairly by a judge. And a lot of people agree with it," Trump said on Fox News.

Trump has been on the defensive since his comments last week about Mexican-American U.S. District Judge Gonzalo Curiel, who is overseeing fraud lawsuits against Trump University, the New York businessman's defunct real estate school.

He has also been castigated by Democrats, including the party's likely presidential nominee Hillary Clinton.

"I don't know what else you could call these attacks other than racist, other than prejudiced, other than bigoted," Clinton said in an interview with MSNBC on Monday. "It's just plain wrong, and certainly wrong coming from someone who is vying to become President of the United States."

Trump told campaign supporters in a conference call they should stand by him in his attacks on the judge, Bloomberg Politics reported. It said during the call Trump distanced himself from a memo his campaign sent supporters on Sunday that urged them to avoid the issue.

Asked for comment, Trump spokeswoman Hope Hicks said: "It was a very positive call to discuss overall messaging."

Two Trump supporters, Tana Goertz and Scottie Nell Hughes, told CNN that former Arizona Governor Jan Brewer had said on the call that she wanted the campaign to go on the offense against expected Democratic nominee Hillary Clinton.

Goertz said Trump told his suppporters not to be afraid to "call out the media" when TV reporters suggest that Trump is racist.

Trump has suggested that Curiel's heritage is influencing the judge's opinion about the case because of Trump's campaign rhetoric about illegal immigration. Trump, the presumptive Republican nominee for the Nov. 8 presidential election, has pledged to seal the U.S.-Mexico border with a wall, and has said Mexico is sending rapists and drug dealers to the United States.

Former Republican presidential candidate Marco Rubio, a U.S. senator from Florida, joined the chorus of criticism. In an interview with Orlando's WFTV, Rubio called Trump's comments wrong and said he had to stop.

"I ran for president and I warned this was going to happen," Rubio said.

Trump has regularly stirred up controversy on the campaign trail and has frequently dismayed Republican establishment leaders. His view of an ethnically biased judiciary has drawn a fresh wave of criticism, including concern in his own party.

On Sunday he was asked if - by the same token - he believed a Muslim judge would be biased against him based on Trump's call for a temporary ban on Muslims entering the country. "It's possible. Yes," Trump said on CBS' "Face the Nation."

Republican leaders including House of Representatives Speaker Paul Ryan and Senate Republican leader Mitch McConnell have distanced themselves from Trump's comments, saying they are worried the tone of his presidential campaign could enrage Latinos, who are a growing U.S. voting bloc.

'BIG TROUBLE'

"If this doesn't change we're in for big trouble," Senator Jeff Flake, an Arizona Republican, said on MSNBC on Monday. "I hope to be able to support the nominee. I certainly can't now," said Flake, whose state has a large percentage of Hispanics.

A former rival for the Republican presidential nomination, Ohio Governor John Kasich, called on Trump to apologize to Curiel, who was born in Indiana to Mexican immigrant parents.

"Attacking judges based on their race (and/or) religion is another tactic that divides our country," Kasich wrote on Twitter. "More importantly, it is flat out wrong."

It was unclear what, if any, effect the latest furor would have on Trump's unorthodox candidacy. States with significant Hispanic populations including California, New Mexico and New Jersey hold nominating contests on Tuesday.

Trump fought back against his Republican critics on Monday, with especially sharp words for Newt Gingrich, a former House speaker.

Gingrich, mentioned as a possible running mate to Trump, on Sunday called Trump's comments inexcusable.

"I was surprised at Newt," Trump told the "Fox & Friends" program. "I thought it was inappropriate what he said."

(Additional reporting by Susan Heavey, Megan Cassella and Steve Holland in Washington, Angela Moon in New York; Editing by Howard Goller and Richard Pullin)