To Donald J. Trump’s campaign, his win in the Iowa caucuses by a record 30-point margin was a sign he would steamroll to the nomination. To hear Gov. Ron DeSantis of Florida tell it, the result was actually a sign of the former president’s weakness.

Mr. DeSantis began offering on Friday a public version of private commentary he has been making: that Mr. Trump’s failure to get much more than roughly 50 percent of the vote during caucuses with the lowest turnout in decades indicates an inability to galvanize the Republican base in a way that signals danger in a general election.

Speaking at a news conference outside the site of a planned debate that was canceled after Nikki Haley, a former United Nations ambassador under Mr. Trump, said she would not take part without her former boss onstage, Mr. DeSantis declared that Mr. Trump’s performance in Iowa was a “warning sign for the party in November.”

“It’s not that it was a weak result to win the caucus,” Mr. DeSantis said. “It’s a question of what does that portend for November and how the Republican base is going to be energized or not energized.”

Turnout in the Iowa caucuses was roughly 110,000 voters. That is about half of what campaigns had once expected, and far below the 186,000 who came out in 2016. “Yes, it was cold,” Mr. DeSantis explained. “Yes, the conditions weren’t great.”

But still.

“The overall danger sign is the low number of people that turned out,” he said.

Mr. DeSantis described Mr. Trump as essentially an incumbent, and said he thought that Mr. Trump, as a former president, should have done better in Iowa — despite Mr. DeSantis’s repeated predictions that he would win there.

“I kind of feel like Reagan would have won 80 percent, if he were — if he were running again,” Mr. DeSantis said.

By contrast, Mr. DeSantis said, Mr. Trump is shedding the support he once had. “There are a class of Republicans that, that voted for him twice, and were happy to do that, who’ve checked out at this point, and it’s not like he’s got votes to spare,” he said.

Minutes earlier, in a Fox News interview with Neil Cavuto, Mr. DeSantis had lamented how “the inevitability thing” was helping Mr. Trump — that voters, even his potential supporters, had begun to write off the contest. Mr. DeSantis said this even as he predicted that Mr. Trump would march through New Hampshire with little resistance.

“Trump will run away with it in New Hampshire — I think that’s pretty clear at this point,” Mr. DeSantis said on Fox News, dismissing Ms. Haley’s chances there and beyond.

Some of the few dozen reporters from the national press corps were listening, huddled around an iPhone streaming the interview and waiting for Mr. DeSantis to begin his outdoor news conference in New Hampshire, where the temperature had dipped into the 20s.

A hearty group of DeSantis fans, some of them without coats, stood behind the prepared stand of microphones. They bounced. They bobbed. They shivered. It had been about an hour.

“I knew it was going to be outside,” one supporter said. “I didn’t know it was going to be this long.”

Mr. DeSantis arrived, wearing a blue suit but no coat, and spoke at length about Mr. Trump’s vote share and enthusiasm for the fall. He talked about his record in Florida. His answers were discursive, and reporters tried to interject. “Stop,” he said, putting a hand up when a reporter tried to interrupt with a question about whether he was running as something of an insurance policy in case Mr. Trump falters.

He declared that he did not want to play pundit — before doing just that.

I’m not a political prognosticator,” he said in response to a question about which state would be the one where he would beat Mr. Trump. “Here’s the deal. Republican voters, we saw in Iowa: Half wanted him, half didn’t, roughly. The turnout was abysmal. There’s terrible enthusiasm right now based off that result.”

Now,” he asked, “looking forward to November, what does that look like for this party if he’s the nominee?”

In a hallmark of Mr. DeSantis’s extensive reliance on an outside group essentially to carry his campaign, the news conference was staged by his super PAC, Never Back Down. Mr. DeSantis was a “special guest.”

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