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Donald J. Trump, whose presidential campaign has doused the Republican Party with the verbal equivalent of napalm, has escalated his rhetoric to new levels of intensity before the crucial South Carolina primary this week. It is a vote that, if he wins, could erode the prospects of Republican leaders’ stopping his candidacy.

During the Republican debate in South Carolina over the weekend, Mr. Trump condemned former President George W. Bush in acid terms, saying his administration had lied about the presence of weapons of mass destruction in Iraq. “They lied,” Mr. Trump said — words that even two Democratic presidential nominees declined to use about Mr. Bush and the 2003 invasion of Iraq, and that were more akin to what the rabble-rouser liberal filmmaker Michael Moore has said.

But Mr. Trump did not stop there: He shredded Mr. Bush as a poor leader who let the World Trade Center come down during his watch. In the process, Mr. Trump both challenged Republican orthodoxy and picked at old, sensitive scabs within the party over the war, all in the final days of the primary campaign in a military-heavy state that has been instrumental to the success of the most dominant family in Republican politics for the last 25 years.

If Mr. Trump wins the primary on Saturday, as every public poll shows him on track to do, it will be after reverting to the storm-the-gates populism that led to his political rise and refusing to back down amid criticism. But his remarks in the debate were a gamble a week from when voters head to the polls, and raised the stakes if he wins: The Republican Party’s eventual nominee could be someone who has been taunting its leaders for months. And Mr. Bush is planning to campaign in South Carolina on Monday for his brother Jeb Bush, who has repeatedly invoked his family as the primary vote approaches, and who appeared to get under Mr. Trump’s skin during the debate. Also on Monday, Mr. Trump has scheduled a rare news conference in South Carolina, as rival campaigns tried to make sense of a multi-candidate battlefield at the debate.

However late he began doing it, Jeb Bush has been the only candidate who has consistently taken on Mr. Trump. In a Sunday afternoon email to supporters, Mr. Bush pressed the case he’ll make over the next week. The subject line said, “Donald crossed a line.”

In a round of interviews and on the Sunday morning programs, Mr. Trump, who has spent the entire campaign defying predictions that his comments would come back to bite him, responded to questions about his comments in the debate with an across-the-board “I don’t care.” He dismissed complaints that he had gone too far, or that his remarks could hurt him in a state where the former president remains highly popular.

“If it does, it does — I have to tell the truth,” Mr. Trump said in an interview with The New York Times.

“I could be very quiet,” he added. “I could say, ‘Oh, that’s wonderful,’ or I could say, ‘Excuse me, the World Trade Center'” came down during Mr. Bush’s tenure.

Mr. Trump headed into the final week in South Carolina with a crescendo of orthodoxy-challenging pronouncements, including criticism of the home-state Senator Lindsey Graham. He also defended his previous comments about the work of Planned Parenthood, saying during the debate, when challenged by Senator Ted Cruz of Texas, that he did not approve of abortion but that the organization did “wonderful work” for women’s health more broadly. That remark could be toxic for Republican primary voters; it had made its way into an ad from Mr. Cruz by late Sunday afternoon.

Mr. Trump’s comments about the former president overshadowed those about Planned Parenthood. But even some former advisers to George Bush did not believe that the remarks about the Iraq war and the World Trade Center would do much damage to Mr. Trump.

“Overwhelmingly, most Republicans disagree with his criticisms of George W. Bush, but most Republicans also don’t want to debate” Mr. Bush’s legacy, said Steve Schmidt, a Republican strategist who was a top adviser to Mr. Bush’s 2004 campaign.

Most people share Mr. Trump’s view that the war was a mistake, Mr. Schmidt added, and in a state heavy with military members, many families have grown weary of repeated deployments.

Tony Fabrizio, a Republican pollster who advised Senator Rand Paul of Kentucky’s presidential campaign, also saw little peril for Mr. Trump, even if one of his opponents puts the remark into a television or radio ad.

“It could potentially bring down his margin, but when you’ve got a guy who wins by double digits, does it matter?” Mr. Fabrizio said.

For Mr. Trump, standing by even his contentious comments has been a hallmark of his appeal.

“What the electorate’s been responding to is strength,” Mr. Schmidt said. “Mr. Trump once again last night showed what many Republican voters will regard as strength.”

More problematic for Mr. Trump, Mr. Fabrizio said, could be his Planned Parenthood comments.

Mr. Trump made noises before the Iowa caucuses about being a “healer” and aligning with the Republican establishment. In a crucial debate in New Hampshire, his approach was to avoid torching his opponents for the most part. But when he had a dust-up with Mr. Bush during the final debate in New Hampshire and the audience booed Mr. Trump, he made the most of it, saying they were against him because they were major donors and he could not be controlled.

He has since used that line repeatedly on the campaign trail, even singling out Mr. Bush’s finance chairman, the billionaire Woody Johnson, as an example of the type of donor who is working against Mr. Trump — never mind that Mr. Trump and Mr. Johnson are actually friends.

When the crowd at the South Carolina debate on Saturday night also began to boo, Mr. Trump again sought to use it to his advantage, saying, “I only tell the truth, lobbyists!” At another point, he criticized Mr. Graham for polling poorly in his presidential race. And he said Mr. Cruz was “the single biggest liar” — something that would have been seen as indecorous in previous campaigns, but that channels the anger voters feel toward elected officials.

That coarsening of the language in the campaign, Mr. Schmidt argued, reflects the toxic culture of social media, and such behavior is no longer considered as much of a risk.

Mr. Trump’s comments about both George Bush and Planned Parenthood are likely to stick to him well beyond the vote in South Carolina, in other primaries. But as for South Carolina, even those who would like to see him disappear from the race struggled to suggest that, after so many predictions of his demise, his latest comment would be the one to prove debilitating. While his margin of victory might be diminished, some operatives suggested, as long as there are multiple candidates, Mr. Trump will continue to amass delegates and roll toward the convention in Cleveland.

“Accusing President Bush of lying about the war adds to the hard cap of Trump’s ceiling,” said Rob Stutzman, a California-based Republican strategist and a supporter of Jeb Bush. “But I don’t think it matters until this is a two-man or three-man race.”

Trygve Olson, who also advised Mr. Paul’s campaign, said, “Conventional wisdom would say Trump should end up paying a high price for his thermonuclear gambit on the debate stage, his attack on President Bush and his positions that are totally incongruent with the vast majority of conservative voters.” But he was reluctant to embrace that wisdom.

“I personally think he will pay a price,” Mr. Olson said, “but given how this cycle is unfolding with regards to Trump, the reality is it is difficult, if not impossible, to say that will be the case.”