“I did two tours in Iraq,” the man said, as the crowd erupted in cheers and chants. “If it weren’t for Mr. Trump right here, I don’t think any of us would have the voice that we have.”

Rival Republican candidates have long criticized Mr. Trump for once holding liberal views on matters like abortion. And conservative commentators have exposed his apostasy on issues like free trade and entitlements.

But nowhere was his break with the party’s orthodoxy more vivid than in the debate last Saturday in South Carolina, a week before Republicans hold their primary here. Besides attacking the former president, while the audience booed loudly, Mr. Trump asserted that, abortion aside, Planned Parenthood did “wonderful things” for women’s health.

Senator Ted Cruz of Texas, who last year threatened to shut down the federal government over funding Planned Parenthood, and who has been hoping to carve evangelical support away from Mr. Trump, quickly went up with a negative ad about Mr. Trump’s support for the group.

But many people at his rallies agreed with Mr. Trump on the issue. “I oppose abortion, but I think Planned Parenthood does a lot of good for people who can’t afford birth control,” said Kim Wells, a schoolteacher and Trump supporter in North Augusta.

At a rally in Louisiana last week, Mr. Trump rejected attacks from Jeb Bush and other candidates that he was not a conservative. He dismissed ideological labels altogether, a sentiment endorsed by the 10,000 people in the arena, who thundered their approval over and over. Instead of calling himself conservative, Mr. Trump said, “I’m a guy with common sense that’s going to make us a fortune.”