When Kim Reynolds, the Republican governor of Iowa, stopped by a donor retreat that Gov. Ron DeSantis of Florida held last year, no one paid it much mind.

When she sat earlier this year with Mr. DeSantis onstage, at another donor gathering down the road from Donald J. Trump’s residence, people began to notice. When she glowingly appeared with Mr. DeSantis not once, not twice, but at all three of his first visits to her state this year, eyebrows arched. And by the time Ms. Reynolds appeared on Thursday alongside Casey DeSantis, the governor’s wife, alarms inside the Trump headquarters were blaring.

Ms. Reynolds has said — including privately, to Mr. Trump — that she does not plan to formally endorse a candidate in the presidential race, in keeping with a tradition that the Iowa governor stays on the sidelines, keeping the playing field level for the first G.O.P. nominating contest. But through her words and deeds, Ms. Reynolds seems to be softening the ground in Iowa for Mr. DeSantis, appearing to try to create the conditions for an opening for him to take on Mr. Trump.

For Mr. DeSantis, Iowa is where his allies acknowledge he must first halt Mr. Trump’s momentum to prevent him from steam-rolling his way to a third consecutive G.O.P. nomination. For Mr. Trump, it is where he hopes to snuff out his challengers’ candidacies, and win where he did not in 2016.

And there is no politician in Iowa with greater sway than Ms. Reynolds, 63, who has overseen her party’s swelling state legislative majorities with an approval rating among Republicans near 90 percent. Republicans say she can command attention and shape the landscape even without making a formal endorsement.

Ms. Reynolds has appeared alongside other candidates — including Mr. Trump, Nikki Haley, Vivek Ramaswamy and Tim Scott — but the warmth of her embrace of Mr. DeSantis has become conspicuous. It has been the subject of internal Trump campaign discussions — it has not escaped their notice that one of her senior political advisers, Ryan Koopmans, is also a top DeSantis super PAC adviser — and even public fulminations from the former president.

“I hate to say it, without me, you know, she was not going to win, you know that, right?” Mr. Trump said of Ms. Reynolds when he campaigned in Iowa in June.

The Republican crowd, notably, did not applaud that off-key remark, which came only months after Ms. Reynolds had romped to re-election, carrying 95 of the state’s 99 counties. But the claim spoke to the former president’s self-centered view of the world: That it was his appointment of her predecessor, Terry Branstad, as his ambassador to China that cleared the way for Ms. Reynolds, then Mr. Branstad’s lieutenant governor, to take the state’s top job.

Ms. Reynolds is said to have tired of Mr. Trump, and she reacted with disbelief to his comment that she owed him her governorship, according to people familiar with her thinking and her response. Still, she sided with Mr. Trump after his most recent indictment, lashing out at the Biden administration and saying it was a “sad day for America.”

The two do have a shared history: Ms. Reynolds narrowly won a full term in 2018 with only 50.3 percent of the vote after Mr. Trump held a late rally for her, hailing her as “someone who has become a real star in the Republican Party.” More recently, however, Mr. Trump has been privately complaining about Ms. Reynolds and other prominent Republicans, who he feels owe him their endorsements because of his past support.

Before Mr. Trump’s latest visit to Iowa on Friday, a behind-the-scenes standoff played out for days over whether Ms. Reynolds would join him. Ms. Reynolds has said she will make an effort to appear with whomever invites her, but an aide said she had not actually been invited. The Trump team sees her as having a standing invitation. Ultimately, she did not attend.

The relationship with Mr. DeSantis, who has privately courted Ms. Reynolds for many months, has been strikingly different.

He calls her Kim.

She calls him Ron.

They banter with a degree of familiarity and friendship that Mr. DeSantis rarely flashes with other politicians. People who know them say they forged a bond during the coronavirus pandemic, as two governors who pressed to open their states over the warnings of some public health officials. They sat down for a private dinner in March, on his first visit to Iowa this year, according to two people briefed on the meal, and in 2022 Mr. DeSantis called Ms. Reynolds to offer his encouragement ahead of her State of the Union response.

When Mr. DeSantis was asked by a local television interviewer on his first trip to Iowa as a presidential candidate if he’d consider Ms. Reynolds for a potential cabinet post, he offered a surprisingly expansive answer, suggestive of something even more lofty: “I mean I think Kim could be considered for just about anything that a president would pick.”

At times, she has had the look of a running mate.

Appearing with Mr. DeSantis at three of his four visits to Iowa this year, and now with his wife as well, Ms. Reynolds has extolled Florida’s achievements under his leadership and connected his state’s successes to Iowa’s. The two lavish compliments on each other, and their talking points echo in perfect harmony.

He says Florida is “the Iowa of the Southeast.” She says Iowa is “the Florida of the North.”

In her introduction at his kickoff event, she made a point of specifically praising Mr. DeSantis for signing a six-week abortion ban, which Mr. Trump has criticized.

“He proudly signed a law that makes it illegal to stop a baby’s beating heart — the same heartbeat bill that I was proud to sign,” she said of Mr. DeSantis.

Some Iowa Republicans said Ms. Reynolds is simply being a gracious host.

“She’s very popular but I don’t think she’s playing favorites,” said Steve Scheffler, one of the state’s Republican National Committee members and the president of the Iowa Faith and Freedom Coalition. “People read way too much into this.”

But Trump advisers have snickered privately about her having neutrality-in-name-only. “She is quote unquote neutral,” said a person close to Mr. Trump, who spoke on condition of anonymity to describe the team’s thinking, which is that Ms. Reynolds will do whatever she can to help Mr. DeSantis short of endorsing him.

The Washington bureau chief for Breitbart News, Matthew Boyle, who is known for his close relationship with the former president, glaringly left Ms. Reynolds off his recent list of 14 Republicans Mr. Trump could pick as his 2024 running mate.

Mr. Trump has some well-placed allies in Iowa — the state party chairman’s son, who is in the legislature, is among his paid advisers — and he is seeking more. On his June visit to the state, he invited a small group of prominent Republican officials whose endorsements are still up for grabs out for dinner at a steakhouse in downtown Des Moines, among them the state’s attorney general, according to people who attended the meal.

In an interview, Mr. Branstad, the former Iowa governor, described the Trump-Reynolds relationship as “cordial,” praised Ms. Reynolds as a popular and effective governor and said her formal neutrality was good for all Iowans. He urged the former president to overcome his irritation.

“Trump has got to get over it,” Mr. Branstad said. “He’s got to get over the jealousy and resentment and focus on the future. You win elections by focusing on the future and not the past.”

There has been no recent independent polling in Iowa. In national surveys, Mr. Trump has led Mr. DeSantis by a wide margin.

Ms. Reynolds is not just the governor of Iowa: She also presides over the Republican Governors Association, the nationwide campaign arm for Republicans seeking governorships. Both her elected G.O.P. counterparts leading the Senate and House campaign arms have already endorsed Mr. Trump.

Yet like other prominent Iowa elected officials, Ms. Reynolds has made it clear that her primary goal is to ensure that Iowa keeps its “first in the nation” status. At a college-football game last fall in Iowa, Ms. Reynolds was in a V.I.P. box mingling with members of the state’s congressional delegation as they discussed the importance of staying “neutral” to protect Iowa’s enviable position at the top of the Republican voting calendar, according to a person present for the conversation (Democrats took away the state’s leadoff spot in 2024).

“We aren’t going to get involved in campaigns, because we want everybody to feel welcome in Iowa,” Senator Chuck Grassley, the 89-year-old Republican senior statesman, said in an interview. “And if the governor were to back somebody, that may discourage other people from coming. Same way for me.”

But there is some burbling frustration with Mr. Trump inside the delegation.

Last month, Mr. Trump skipped the signature “Roast and Ride” event organized by Senator Joni Ernst of Iowa. His campaign had expressed interest in sending videotaped remarks, and Ms. Ernst’s operation then rented large screens for the purpose of showing them, but he never sent a video — leaving Ms. Ernst’s team without a recording, and the cost of the equipment to cover, according to five people briefed on the incident.

Ms. Ernst’s team had planned on using the chance to win a motorcycle helmet signed by all of the Republican candidates as a lure to sell tickets to the “Roast and Ride.” They sent the helmet to Mr. Trump, who returned it later than expected and had added the numbers “45” and “47,” signaling he would be the 47th president, the role everyone else is also running for, according to two people with knowledge of the episode. They never used the helmet.

In March of this year, Ms. Reynolds did introduce Mr. Trump at an event. In a private meeting during that same trip, Ms. Reynolds stressed to Mr. Trump that her focus was on maintaining Iowa’s place as the first state in the nation on the campaign calendar, according to a person familiar with what took place but who was not authorized to discuss it publicly. Mr. Trump responded by telling her that he was the one who had protected the caucuses’ leadoff position, as president. (The Iowa caucuses have begun the nominating process since the 1970s.)

At their joint event on Thursday, Ms. Reynolds and Ms. DeSantis bantered onstage and even exchanged a high-five.

“I am a woman on a mission,” Ms. Reynolds said at one point, “and I think you are a woman on a mission, too.”

Lisa Lerer contributed reporting.