At a packed community center in southwestern Iowa, Nikki Haley broke from her usual remarks this month to offer a warning to her top Republican presidential rivals, Donald J. Trump and Ron DeSantis, deploying a favorite line: “If they punch me, I punch back — and I punch back harder.”

But in that Dec. 18 appearance and over the next few days, Ms. Haley, the former governor of South Carolina, did not exactly pummel her opponents as promised. Her jabs were instead surgical, dry and policy-driven.

“He went into D.C. saying that he was going to stop the spending and instead, he voted to raise the debt limit,” Ms. Haley said of Mr. DeSantis, a former congressman, in Treynor, near the Nebraska border. At that same stop, she also defended herself against his attack ads and criticized Mr. DeSantis, the Florida governor, over offshore drilling and fracking, and questioned his choice of a political surrogate in Iowa.

She was even more careful about going after Mr. Trump, continuing to draw only indirect contrasts and noting pointedly that his allied super PAC had begun running anti-Haley ads.

“He said two days ago I wasn’t surging,” she said, but now had “attack ads going up against me.”

With under three weeks left until the Iowa caucuses, Ms. Haley is treading cautiously as she enters the crucial final stretch of her campaign to shake the Republican Party loose from the clutches of Mr. Trump. Even as the former president maintains a vast lead in polls, Ms. Haley has insistently played it safe, betting that an approach that has left her as the only non-Trump candidate with any sort of momentum can eventually prevail as primary season unfolds.

On the trail, she rarely takes questions from reporters. She hardly deviates from her stump speech or generates headlines. And she keeps walking a fine line on her greatest obstacle to the Republican nomination — Mr. Trump.

“Anti-Trumpers don’t think I hate him enough,” she told reporters this month in New Hampshire, where she picked up the endorsement of Chris Sununu, the state’s popular Republican governor. “Pro-Trumpers don’t think I love him enough.”

Ms. Haley’s consistent strategy has enabled her team to build a reputation as lean and stable where other campaigns have faltered: As Mr. DeSantis’s support has dipped and turmoil has overtaken his allied super PAC, even some of his advisers are privately signaling they believe hope is lost.

“I keep coming back to the word ‘disciplined,’” said Jim Merrill, a Republican strategist in New Hampshire who served on Senator Marco Rubio’s 2016 presidential campaign and Mitt Romney’s 2008 and 2012 bids. “She has run an extraordinarily disciplined campaign.”

Yet Mr. Trump remains the heavy favorite for the nomination despite facing dozens of criminal charges, as well as legal challenges that aim to kick him off the ballot in several states.

Ms. Haley’s apparent reluctance to attack her rival even in the face of what would seem to be political setbacks for him has raised questions from voters and other Republican competitors — most notably, former Gov. Chris Christie of New Jersey — about whether she can win while passing up crucial opportunities to derail her most significant opponent.

“A lot of the people in this field are running against Trump without doing very much to take him on,” said Adolphus Belk, a political analyst and professor of political science at Winthrop University in Rock Hill, S.C., Ms. Haley’s home state. “If you are running to be president of the United States, it seems like it would be an imperative to take on the person who has the biggest lead.”

A recent poll from The New York Times and Siena College found Mr. Trump leading his Republican rivals by more than 50 percentage points nationally, a staggering margin.

The poll offered a sliver of hope for Ms. Haley: Nearly a quarter of Mr. Trump’s supporters said he should not be the Republican nominee if he were found guilty of a crime. But 62 percent of Republicans said that if the former president won the primary, he should remain the nominee — even if subsequently convicted.

The challenge for Ms. Haley is peeling away more of his support from the Republican Party’s white, working-class base. The Times/Siena poll found that she garnered 28 percent support from white voters with a bachelor’s degree or higher, but just 3 percent from those without a degree.

As she barnstorms through Iowa and New Hampshire, Ms. Haley has remained committed to a calibrated approach that aims to speak to all factions of the Republican Party.

Her stump speech highlights her background as the daughter of immigrants and her upbringing in a small and rural South Carolina town, but in generic terms. She nods to her status as the only woman in the Republican primary field and the potentially historic nature of her bid, but only in subtle ways.

Even as she has risen in the polls and consolidated significant anti-Trump support among donors and prominent Republicans, she has continued to cast herself as an underestimated underdog, with a message tightly focused on debt and spending, national security and the crisis at the border.

And she has not strayed from her broad calls for a “consensus” on abortion, even though some conservatives say she is not going far enough in backing new restrictions. At the same time, Democrats are looking to hit her from the other direction: The Democratic National Committee last week put up billboards in Davenport, Iowa, where she was campaigning, accusing her of wanting “extreme abortion bans.”

Still, Ms. Haley has evolved on some fronts. In recent weeks, she has more aggressively made the case that she is the most electable Republican candidate — an argument that polls show has some merit — and ramped up her critiques of what she describes as a dysfunctional Washington.

This month, after Republicans blocked an emergency spending bill to fund support for Ukraine, demanding strict new border restrictions in return, she accused both President Biden and some Republicans of creating a false choice among those priorities, as well as aid to Israel, which the legislation also included.

“And now what are you hearing coming out of D.C. — do we support Ukraine or do we support Israel?” she said at an event in Burlington, Iowa. “Do we support Israel or do we secure the border? Don’t let them lie to you like that.”

She has ramped up her criticism of Mr. Trump on his tone, leadership style and what she describes as his lack of follow-through on policy, hitting him for increasing the national debt, proposing to raise the federal gasoline tax and “praising dictators.”

But when confronted with tougher questions from voters over Mr. Trump’s potential danger to the nation’s democracy or why she indicated at the first debate that she would support him as the nominee even if he were convicted of criminal charges, she tends to fall back on a familiar response. She says she thinks that “he was the right president for the right time” but that “rightly or wrongly, chaos follows him.”

“The thing is, normal people aren’t obsessed with Trump like you guys are,” she told Jonathan Karl of ABC News this month, taking a swipe at the news media when asked for her thoughts on how Mr. Trump is campaigning on the idea of “retribution” against his political enemies.

Such attempts to avoid alienating Trump supporters have helped generate interest, if not always commitment.

Before her event in Treynor, Iowa, Keith Denton, 77, a retired farmer and longtime Republican, said he stood with Mr. Trump “100 percent,” and had come to watch Ms. Haley only because his wife was debating whether to support her. But after Ms. Haley wrapped up, he tracked down a reporter to acknowledge that he was now seriously considering her.

“I have to eat my words,” he said, adding that Ms. Haley had said “some things that changed my mind.” For one, he said, “I thought she was more of a warmonger, but now I can see she is against war.”

But at an Osceola distilling company the next day, Jim Kimball, 84, a retired doctor, veteran and anti-Trump Republican, elicited nervous laughter from the audience when he asked Ms. Haley a couple of bold questions regarding the Capitol riot on Jan. 6, 2021: “Did Mr. Trump trample or defend the Constitution? And is he running for president or emperor?”

As usual, Ms. Haley weighed her words. She said that the courts would “decide whether President Trump did something wrong” and that he had a right to defend himself against the legal charges he faces, but she expressed disappointment that when he had the chance to stop the Capitol attack, he did not.

“My goal is not to worry about him being president forever — that is why I’m going to win,” she finished to loud applause.

But afterward, Mr. Kimball said that he wished she would have said that Mr. Trump is unfit to be president and that he was still deliberating whether to caucus for her or for Mr. Christie.

“I wish she had the courage of Liz Cheney,” he said, referring to the congresswoman pushed out of Republican leadership in Congress and then her Wyoming seat by pro-Trump forces in the party. “But she doesn’t want to end up like Liz Cheney, so you get the answer you get.”

Ruth Igielnik contributed reporting.