In an unusually coordinated series of attacks leveled from congressional offices and the Senate floor, in state capitols and sidewalk protests, Democrats excoriated Mr. Trump as racist and demanded that Republicans either stand behind his comments or condemn him and even rescind their endorsements of his candidacy.
Democrats received unexpected ammunition from Mr. Trump himself, who, in an extraordinary conference call with allies on Monday, urged them to defend his criticisms of a federal judge’s Mexican heritage — and then rebuked his campaign staff for having suggested otherwise.
Mr. Trump’s doggedness, and his chastisement of his own aides, contributed to a sense of powerlessness among Republicans who said they increasingly saw no way to influence Mr. Trump’s behavior or to convince him that his actions could hurt the party in competitive House, Senate and governor’s races.
Reince Priebus, the chairman of the Republican National Committee, has held discussions with Mr. Trump about his attacks on Judge Gonzalo P. Curiel of Federal District Court, who is overseeing a suit against the now-shuttered Trump University, according to a Republican briefed on the talks.
But Mr. Priebus has had similar conversations over many months with Mr. Trump, to little avail. And other senior Republicans said there was confusion about whether it was worth approaching any of Mr. Trump’s aides about doing an intervention with him.
Republican candidates in tough races this fall were left to fend for themselves. Senator Kelly Ayotte of New Hampshire called on Mr. Trump to retract his comments about Judge Curiel, calling them “offensive and wrong.”
Other candidates lay low. Senator Marco Rubio of Florida, a former rival of Mr. Trump’s, and Senator Susan Collins of Maine both criticized Mr. Trump, yet neither they nor Ms. Ayotte indicated that they would reject his candidacy.
“I continue to hope that Mr. Trump will rethink his position and take back those words and show respect for the separation of powers doctrine that is enshrined in our Constitution,” Ms. Collins said in an interview. “I continue to believe in redemption.”
No prominent elected Republican came to Mr. Trump’s defense unreservedly. And others found themselves wondering aloud what it would take — what Mr. Trump would have to say or do — for Republicans who have endorsed him to start jumping ship.
Senator Lindsey Graham of South Carolina, another former primary rival of Mr. Trump’s, urged Republicans who have backed Mr. Trump to rescind their endorsements, citing the remarks about Judge Curiel and Mr. Trump’s expression of doubt on Sunday that a Muslim judge could remain neutral in the same lawsuit, given Mr. Trump’s proposed ban on Muslim noncitizens entering the country.
“This is the most un-American thing from a politician since Joe McCarthy,” Mr. Graham said. “If anybody was looking for an off-ramp, this is probably it,” he added. “There’ll come a time when the love of country will trump hatred of Hillary.”
Republican hopes that Mr. Trump would tone down his language and heed advice were also undercut after he laced into his own campaign team during a strategy discussion with political surrogates.
On the conference call, reported first by the Bloomberg News website and later confirmed by two participants, Mr. Trump argued that talking points sent by his campaign to his allies, directing them not to discuss the Trump University case, were ill advised.
“Take that order and throw it the hell out,” Mr. Trump said.
He then went further in excoriating his campaign staff: “Are there any other stupid letters that were sent to you folks?” Mr. Trump said, according to the Bloomberg News report. “That’s one of the reasons I want to have this call, because you guys are getting sometimes stupid information from people that aren’t so smart.”
No other modern presidential campaign has unfolded like this, and gleeful Democrats have concluded that one of their best strategies for the general election is to hold Republicans accountable for each new Trump bombshell.
In Indiana, state Democratic Party officials held a news conference on Monday to assail Mr. Trump’s remarks about Judge Curiel, who was born in the state and graduated from Indiana University’s law school. But the party’s chairman, John Zody, spent even more time criticizing Gov. Mike Pence, a Republican seeking re-election this year, and Republican House and Senate candidates in Indiana for being “completely silent on the fact that Donald Trump is questioning the integrity of a Hoosier.” Only a few Republican officials in the state have criticized Mr. Trump’s comments as inappropriate.
Outside Trump Tower in Manhattan, meanwhile, Melissa Mark-Viverito, the speaker of the New York City Council, and Councilman Carlos Menchaca told reporters that Mr. Trump’s comments about the judge were racist and xenophobic and reflected disrespect for the judiciary system.