Gov. Ron DeSantis of Florida needed a lift in his quest to beat former President Donald J. Trump in the crucial Iowa caucuses.
On Monday, Mr. DeSantis may have gotten one, as he received the endorsement of Kim Reynolds, the state’s popular Republican governor, during a recorded interview on NBC News.
“I just felt like I couldn’t sit on the sidelines any longer,” Ms. Reynolds said. “We have too much at stake. I truly believe that he is the right person to get this country back on track.”
Mr. DeSantis, sitting beside her, called her support in the Republican presidential primary “very meaningful.”
The two governors then appeared together at a campaign rally in Des Moines, where Ms. Reynolds proclaimed that the nation needed a leader “who puts this country first and not himself” — a clear jab at Mr. Trump, whom she did not refer to by name during her remarks.
Before the endorsement, Mr. Trump repeatedly criticized Ms. Reynolds, who had joined Mr. DeSantis at campaign events around Iowa, for her perceived disloyalty. Those personal attacks had outraged the Iowa governor. She had previously said she would stay neutral during the caucuses, as is traditional for sitting governors in the state.
Mr. Trump, meanwhile, spent Monday testifying in a civil fraud trial that threatens his business empire in New York. It’s a contrast that is likely to become a running theme as the primary plays out, with Mr. Trump forced to spend time away from the campaign trail in order to deal with the four criminal indictments against him.
In Iowa, Mr. DeSantis is in need of a jolt. He has staked his campaign on winning the Jan. 15 caucuses, moving much of his staff to the state in a bid to stop Mr. Trump’s momentum. But his poll numbers there have slipped. He is now tied for second place with former Gov. Nikki Haley of South Carolina at 16 percent, far behind Mr. Trump at 43 percent, according to a Des Moines Register/NBC News/Mediacom poll. Mr. DeSantis’s allies point out that the last three G.O.P. winners of the Iowa caucuses were also lagging at this stage of previous election cycles.
Given Mr. DeSantis’s standing in the polls, the endorsement has risks for Ms. Reynolds. But she expressed confidence at the rally that he could win both the primary and the general election against President Biden.
Because of her wide appeal to Iowa Republicans, Ms. Reynolds has the potential to shake up the race. “She’s the reason we’re red,” said Gloria Mazza, the chairwoman of the Republican Party of Polk County, which includes Des Moines. (Ms. Mazza is staying neutral in the caucuses.)
Ben Jung, an undecided Iowa voter, said he attended Monday’s rally, held at an event space near downtown Des Moines, because of Ms. Reynolds’s endorsement.
“It’s prompted me to finally get my feet out here and listen and look a little more closely,” said Mr. Jung, 53, who had been leaning toward supporting Senator Tim Scott of South Carolina but said he was also considering Mr. DeSantis and Ms. Haley.
Leaders of Iowa’s evangelical community, an important voting bloc, suggested Ms. Reynolds’s endorsement was a major coup for the DeSantis campaign.
Ms. Reynolds is “without question Iowa’s most popular Governor in generations,” Bob Vander Plaats, the president and chief executive of the influential Christian conservative group the Family Leader, wrote on X, formerly known as Twitter. “Combine her popularity with her campaign tenacity and she will be a force for” Mr. DeSantis.
But Mr. Trump’s followers have demonstrated their loyalty so far during the primary, even as he faces criminal charges.
Steven Cheung, a spokesman for the Trump campaign, disparaged Ms. Reynolds’s endorsement as a “Hail Mary pass at the end of a blowout game.”
“A Kim Reynolds endorsement does not mean anything and does not sway voters,” Mr. Cheung said in a statement.
During the NBC News interview, Mr. DeSantis took the opportunity to return fire to the former president.
“With Donald Trump, if you don’t kiss the ring, you could be the best governor ever and he’ll trash you,” Mr. DeSantis said. “You could be a terrible, corrupt politician, but if you kiss his ring then all the sudden he’ll praise you.”
Mr. DeSantis has not always been known for his personal touch. Members of Congress who have endorsed Mr. Trump have described the Florida governor as distant. And he has sometimes appeared awkward on the campaign trail.
But in an interview with The Des Moines Register on Monday, Ms. Reynolds praised Mr. DeSantis and his wife, Casey, a breast cancer survivor, for calling her after her husband, Kevin, learned he had lung cancer.
“Not only is he tough and disciplined,” Ms. Reynolds said, “but he’s compassionate and cares.”