“Everybody in the establishment misunderstands the game he’s playing,” said Newt Gingrich, the author of the Contract With America and onetime House speaker who was himself a Republican presidential candidate in 2012. “His opponents want to talk about policies. He’s saying if you don’t have a leader capable of cutting through the baloney, all this policy stuff is an excuse for inaction.”

Anticipating the debate on Wednesday, Mr. Trump said, “I’d rather just discuss the issues.” But he added in an interview on ABC’s “Good Morning America” that he would respond in kind if targeted by a rival. “If I’m attacked, I have to, you know, do something back, but I’d like it to be very civil,” he said.

Waffling, flip-flopping and inconsistencies, all of which might hobble a conventional candidate, have not dimmed Mr. Trump’s appeal to his Republican supporters.

He seemed to lose no ground as rivals and the news media pointed out the stark reversal in his ideology since he flirted with a presidential run in 1999. Back then, Mr. Trump supported abortion rights and a soak-the-rich tax on fortunes in excess of $10 million.

When another presidential contender, former Gov. Rick Perry of Texas, called him “a cancer on conservatism” last month, pointing out Mr. Trump’s previous advocacy for single-payer health care and his support of Hillary Rodham Clinton, it was Mr. Perry, castigated on social media, who paid a price.

A senior adviser to Mr. Perry, Sam Clovis, the chairman of his campaign in Iowa, called Mr. Trump’s appeal “a cult of personality,” and faulted the news media for focusing on his inflammatory remarks and insults rather than on the substance of his candidacy. (On Tuesday, an online news site, Independent Journal Review, posted a compilation of video clips showing Mr. Trump delivering insults; it was 10 hours long.)