Torn campaign signs litter the ground behind a police skirmish line on June 2 during a protest near where Donald Trump held a rally in San Jose, Calif. (JOSH EDELSON/AFP/Getty Images)

Donald Trump, whose campaign for president has crashed through one barrier after another, has again moved his candidacy into highly questionable territory, threatening to stir more racial animosity in an already divided country and putting at risk his party’s relationship with the nation’s fastest-growing minority group.

Over the past week, Trump has repeatedly cited the Mexican heritage of U.S. District Judge Gonzalo Curiel, who is overseeing a lawsuit brought by former students of Trump University alleging fraud against the institution. Trump accused Curiel of a conflict of interest in hearing the case because, according to his reasoning, the judge’s Mexican heritage puts him at odds with Trump’s proposal to build a wall on the U.S.-Mexican border.

When challenged to explain why he believed the judge was biased, Trump said, among other things, “He’s a Mexican.”

Curiel is an American, born in 1953 in Indiana to parents who were Mexican immigrants. The judge has not publicly expressed an opinion about Trump’s proposed wall.

Politicians and others across the ideological spectrum have rebuked Trump in the past for “dog whistle” politics, or worse. In this case, the condemnations have been swift and cutting, as Republican leaders scramble to protect their party against charges that their presumptive presidential nominee is engaging in a racist attack against a sitting judge.

The campaign of the presumptive Republican nominee, Donald Trump, is sparking a surge in the number of citizenship applications and voter registrations among Hispanics fearful of his immigration policies. Since January, California alone has seen a boost of 218 percent in Democratic registration and among Hispanics, registration is up 123 percent. (Alice Li/The Washington Post)

“I don’t know what Trump’s reasoning was, and I don’t care,” former House speaker Newt Gingrich, who has been supportive of Trump, said in an email. “His description of the judge in terms of his parentage is completely unacceptable.”

Throughout the campaign, Trump has seemed immune from the typical effects of this kind of rhetoric and behavior. He has repeatedly survived, even thrived, after making controversial statements, whether attacking former president George W. Bush as a liar who took the country to war in Iraq under false pretenses or claiming that Sen. John McCain (R-Ariz.), a prisoner in the Vietnam War who was tortured repeatedly, was not a hero because he had been shot down and captured.

Those are just two of a number of instances in which Trump has insulted politicians, private citizens or entire groups of people. Based on the record, it is premature to draw conclusions about how his attacks on Curiel will affect him politically. But they have prompted deepening concern among others in the Republican Party that he is pursuing a strategy — if it actually is a strategy — with worrisome ­longer-term consequences for the country and the GOP.

The 2016 presidential campaign is playing out against a backdrop of heightened racial, ethnic and cultural tensions. Violence at Trump rallies, which flared again Thursday in San Jose, has sometimes pitted angry anti-Trump demonstrators, many of them Hispanics, against the candidate’s equally passionate, predominantly white supporters. At one time or another, each side has been responsible for inciting the violence that ensued.

One issue that has inflamed the national debate is police shootings of unarmed African Americans and the relationship between police departments and the black community. Another is the question of how the country should deal with threats of domestic terrorism from Islamic radicals and whether there should be new restrictions on those seeking entry into the United States from countries in the Middle East.

Trump’s candidacy has been fueled by anger over illegal immigration and by rhetoric that began with the opening days of his candidacy, when he said many of those coming to the United States from Mexico illegally were “rapists, . . . murderers” and criminals. Later, after the terrorist attacks in Paris and San Bernardino, Calif., Trump called for a ban on all Muslims seeking to enter the country until the federal government could put in place a more effective system for background screening.

In this latest instance, Trump vehemently denied a charge from Hillary Clinton that he had launched a racist attack against the judge. But his behavior was enough to prompt some of the most prominent members of his party to call him out.

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On Friday, just one day after announcing his support for Trump, House Speaker Paul D. Ryan (R-Wis.) denounced the New York billionaire during an interview with WISN radio in Wisconsin. He said Trump’s accusations against the judge had come “completely out of left field,” adding, “It’s reasoning I don’t relate to. I completely disagree with the thinking behind that.”

Senate Majority Leader Mitch McConnell (R-Ky.) criticized Trump for an earlier attack on New Mexico Gov. Susana Martinez, a Republican, and expressed his concern that Trump’s language and behavior could permanently alienate Hispanics from the Republican Party in much the same way that 1964 Republican presidential nominee Barry Goldwater’s opposition to the Civil Rights Act of 1964 helped turn African Americans into the most loyal voting block in the Democratic coalition.

Gingrich issued a warning to the presumptive GOP nominee to stop freelancing and begin listening to advisers and others about how to run his general-election campaign. “If Trump doesn’t start consulting and coordinating with his allies, he will not have any,” Gingrich wrote in the email.

Bruce Buchanan, a professor of government at the University of Texas, emailed: “There is a long history of race-based unhappiness with court decisions (e.g. George Wallace’s tirades against desegregation orders). But Trump’s rhetoric amounts to a uniquely personal attack on a federal judge that signals a decidedly ‘un-presidential’ disrespect for the legal process. This cannot help his candidacy.”

John Weaver, who served as chief strategist to the unsuccessful presidential campaign of Ohio Gov. John Kasich (R), said in an email, “Trump’s unwarranted and unhinged attacks on a fine American public servant are echoes from this nation’s demagogic past. . . . There is no stopping him from being our nominee, sadly, but this type of racially charged rhetoric will ensure he is adrift and alone before he is defeated in November.”

There have been other episodes in which Trump has veered into the controversy over issues involving race. Last winter, ahead of a crucial round of Southern primary contests, he refused, in an interview on CNN, to denounce former Ku Klux Klan leader and one-time Louisiana politician David Duke or to disavow any expressions of support from Duke or white supremacist organizations.

Trump’s hesitation to speak out forcefully in that interview prompted a parade of Republican elected officials to publicly criticize him in an effort to insulate the party from any fallout. Trump defended himself, pointing to an earlier statement disavowing Duke’s support, but he could not explain satisfactorily why he had not repeated that disavowal during the CNN interview.

When he was exploring a possible 2012 presidential campaign, Trump raised the issue of whether President Obama was born in the United States, a discredited charge that nonetheless was accepted as true by a fifth of the population nationally and by a somewhat higher percentage of Republicans.

Asked in 2013 whether he believed he had carried the “birther” issue too far, Trump said in an interview with ABC’s Jonathan Karl, “I don’t think I went overboard. Actually, I think it made me very popular. . . . I do think I know what I’m doing.”

In his race this year, Trump repeatedly raised the issue of whether Sen. Ted Cruz (R-Tex.) was ineligible to serve as president because he had been born in Canada to a Cuban-born father and American mother.

Trump has brushed aside criticism that he could so alienate Hispanic voters that he would have no chance of winning in November. He has boasted throughout the campaign that he will do far better among Hispanics and African Americans than current polling suggests and better than Mitt Romney did with those voters four years ago.

Exit polls showed that Romney got 27 percent of the Latino vote and 6 percent among African Americans. Current polling shows Trump getting anywhere from 10 or 12 percent of the Hispanic vote to about 30 percent.

This is hardly the first time Trump has found himself the target of criticism for crude and insensitive language. Campaigning Friday in California, he pointed at one person in the audience and said, “Look at my African American over here.”

Early in the campaign, he was taken to task for retweeting from his official Twitter account a racially tinged comment about Jeb Bush’s wife, Columba, who was born in Mexico. The original tweet said, “#JebBush has to like the Mexican Illegals because of his wife.” Trump said he had not personally done the retweeting. The retweet was soon removed from Trump’s feed.

The unfolding case of Trump challenging Curiel began at a rally a week ago when he delivered a lengthy commentary about the Trump University case. He called Curiel a “hater of Donald Trump” and also said that the judge “happens to be, we believe, Mexican.” On Thursday, in an interview with the Wall Street Journal, he charged that Curiel should not be handling the case because of obvious bias. “I’m building a wall,” Trump said. “It’s an absolute conflict of interest.”

On Friday, Trump was pressed repeatedly by CNN’s Jake Tapper to explain the link between the fact that Curiel’s parents were Mexican immigrants and the judge’s ability to handle the case involving Trump University. “I’ve been treated very unfairly by this judge,” Trump said. “Now, this judge is of Mexican heritage. I’m building a wall, okay? I’m building a wall.”

Trump continued to spar with Tapper, who eventually said, “If you are saying he can’t do his job because of his race, is that not the definition of racism?” Trump replied, “No, I don’t think so at all.”

The question and the response were reminders that Trump continues to go where no major-party nominee in recent history has gone.