Republican US presidential candidate Donald Trump arrives at a campaign rally at Richmond Coliseum in Richmond, Virginia, US, June 10, 2016. [Photo/Agencies]

WASHINGTON -- US presumptive Republican presidential nominee Donald Trump is taking heat yet again, this time for some controversial comments against a judge, as experts said this will hurt him in the presidential race.

The brash New York billionaire is embroiled in a lawsuit over Trump University, as some students claimed they did not get their money's worth.

Trump has sharply criticized Gonzalo P. Curiel for being biased against him because the judge is Mexican American and Trump plans to build a wall on the Mexican border to keep illegal immigrants from entering the United States.

Trump's provocative remarks created a firestorm last week as many Democratic and Republican politicians alike dismissed them as racist.

"This issue is hurting Trump," Darrell West, vice president and director of governance studies of the Brookings Institution, told Xinhua.

"He basically said that an American judge of Mexican heritage could not be fair due to political stances that Trump has taken," West said.

Indeed, other Republicans have also blasted Trump over the issue, including key Republican and 2012 contender for the Republican Party nomination Newt Gingrich.

West said it is unprecedented for a major American presidential candidate to criticize a judge based on that individual's ethnicity.

Indeed, such comments will do little to endear Trump to Hispanics, a key voting bloc in the US that is usually in the tank for Democrats.

"On top of all the other things Trump has said, it is hard to envision him getting more than 20 percent of the Hispanic vote," West said.

As most Latinos find Trump offensive and disagree with him on immigration and border security, the vast majority of them will end up voting Democratic and there is little Trump can do to change that, West said.

Meanwhile, Democratic presumptive nominee Hillary Clinton is ramping up her game after clinching the nomination.

While the former secretary of state has often appeared to be stiff and unapproachable in public, Clinton earlier this month unleashed a blistering attack against Trump that grabbed media attention.

Clinton lambasted Trump in a foreign policy speech in the state of California, calling the New York mogul a "fraud" who is "temperamentally unfit" to be a president.

Observers said this was a departure from Clinton's usual stiff public persona, which has been a thorn in her side in her efforts to portray herself as someone who understands the struggles of ordinary Americans.

West said that Clinton is displaying more passion in attacking Trump, and the recent scandal over Trump University gives Clinton an opening to characterize Trump as "a rich guy taking advantage of poor people, West said.

Trump's whirlwind entrance into the US political scene last summer has sparked controversy nationwide and has led to a number of riots at political events, most of which saw violence by protesters against Trump supporters.

Last week, Trump supporters were attacked by a mob at a Trump rally in the state of California, with angry assailants hurling eggs and throwing punches at Trump supporters. Some protesters in that and other anti-Trump riots were seen waving Mexican flags. Anti-Trump protesters also attacked police last month during a riot in California.