Donald J. Trump’s escalating attacks on Nikki Haley both on the airwaves and at his rallies — criticisms she likened Saturday to “a temper tantrum” — captured the turbulent dynamics in the final week before the first votes of the 2024 Republican presidential primary are cast.

Mr. Trump, Ms. Haley and Ron DeSantis fanned out across Iowa this weekend to make their case before the state’s caucuses on Jan. 15 in a frenetic burst of activity as voters endured an unending barrage of mailers, TV ads and door knockers.

But the late gust of campaigning belies a Republican race that has remained stubbornly static for months despite unfolding under the most extraordinary of circumstances. Mr. Trump remains the party’s prohibitive front-runner, even as he stares down legal jeopardy in the form of 91 felony counts spread across four criminal cases.

For months, the date of the Iowa caucuses has been circled on Republican calendars as the first and one of the best opportunities for those hoping to slow Mr. Trump’s march toward a rematch with President Biden. Iowa Republicans, after all, were some of the few voters in the party to reject Mr. Trump in the 2016 primary.

But the former president’s two top rivals — Ms. Haley, the former United Nations ambassador, and Mr. DeSantis, the Florida governor — continue to thrash each other as much as Mr. Trump, though both are badly trailing him in most polls.

The leading pro-Haley super PAC has spent more than $13 million attacking Mr. DeSantis in Iowa since December, including one recent mailer that features Mr. Trump’s distinctive blond hair photoshopped onto Mr. DeSantis, calling the governor “unoriginal” and “too lame to lead.” A pro-DeSantis super PAC, meanwhile, has funded more than $8 million worth of attacks in Iowa on Ms. Haley since November, with ads calling her “Tricky Nikki Haley” and condemning her positions on China and transgender rights.

“It’s literally a circular firing squad for second place,” said Terry Sullivan, a Republican strategist who managed Senator Marco Rubio’s 2016 campaign. “Trump is the de facto incumbent nominee of the party, and if you want to beat an incumbent, you have to give a fireable offense. Their effort has been abysmal at delivering a fireable offense.”

On the third anniversary of the Jan. 6, 2021, riot at the Capitol on Saturday, Mr. Trump indulged in the same lies about the results of the last election that were at the center of the violent uprising, and described those imprisoned for their roles in the attack as “J6 hostages.” But his leading G.O.P. rivals, ever wary of crossing a Trump-aligned party base even as the election nears, left the anniversary mostly unremarked upon. And it was Mr. Biden who on Friday used the occasion to pitch Mr. Trump as unfit for the presidency.

Chris McAnich, who was at Mr. Trump’s event in Newton, Iowa, on Saturday wearing his white “Trump Caucus Captain” hat, said he had specifically attended because of the Jan. 6 date.

“He did not incite a riot, and that’s kind of why I’m here, on Jan. 6, to say I’m with Trump and stick a thumb in their eye,” Mr. McAnich said.

A confident Mr. Trump continued to throw punches at a range of Republicans, including the late Senator John McCain, a former prisoner of war whom Mr. Trump infamously mocked in 2015 when he said, “I like people who weren’t captured.” In Newton on Saturday, Mr. Trump brought up Mr. McCain’s vote against repealing the health care law known as Obamacare.

“John McCain for some reason couldn’t get his arm up that day,” Mr. Trump said, mimicking Mr. McCain’s thumbs-down gesture. Mr. McCain had sustained injuries during his imprisonment that limited his arm mobility.

Entering 2024, Ms. Haley appeared to be gaining momentum, consolidating support among more moderate Republicans. She announced this week that she had hauled in $24 million in the fourth quarter, a major infusion of cash at a critical juncture. The political network founded by the industrialist Koch brothers said it was plunging another $27 million into aiding Ms. Haley, including the first spending in Super Tuesday states.

But she has made some verbal stumbles in recent days as a brighter spotlight shines on her. She suggested that New Hampshire would “correct” Iowa’s vote and that “you change personalities” as the calendar turns to the second voting state, miscues that Mr. DeSantis’s operation hopes he can capitalize on as the battle for second place has raged in Iowa. The DeSantis campaign was texting the quotes to Iowans over the weekend.

Mr. Trump slashed at Ms. Haley, much as he has Mr. DeSantis, for daring to run against him after she said she would not. “Nikki would sell you out just like she sold me out,” Mr. Trump said on Saturday. The day before, he accused her of being “in the pocket” of “establishment donors,” and of being a “globalist.”

“She likes the globe,” Mr. Trump said. “I like America first.”

Mr. Trump’s pivot to Ms. Haley after months of unrelenting attacks on Mr. DeSantis signaled a new phase in the race. Ms. Haley is threatening not only to eclipse Mr. DeSantis for second place in Iowa but also to compete with Mr. Trump in New Hampshire, where independent voters are giving her a lift in a state with an open primary.

Since mid-December, Mr. Trump’s super PAC has spent more than $5 million hitting Ms. Haley in New Hampshire — after spending nothing, federal records show. Mr. Trump’s campaign is now on the airwaves there, too.

“Isn’t that sweet of him spending so much time and money against me?” Ms. Haley said on Fox News on Friday after she was shown a Trump ad attacking her on immigration.

Gov. Chris Sununu of New Hampshire, who has endorsed Ms. Haley and campaigned with her in Iowa this week, said in an interview that Mr. Trump was “scared.”

“He’s seeing exactly what we’re seeing,” Mr. Sununu said. “She’s moving. He’s not. She has momentum. He doesn’t. She’s getting people excited. He’s yesterday’s news.”

Mr. Trump’s team is hoping that a string of early and decisive victories, starting in Iowa and then in New Hampshire, Nevada and South Carolina, will help make him the presumptive nominee by March, when most of the delegates he needs to secure the nomination are up for grabs. The former president has reliably led in national polling by landslide margins for many months. The indictments at the center of Mr. Trump’s legal vulnerability have so far served only to strengthened him politically, with Republicans consistently rallying to his defense.

Mr. Trump’s advisers have said that, in some ways, they are battling complacency as much as they are his rivals, with surveys showing him so far ahead. “Don’t go by the polls,” Mr. Trump said on Saturday, urging Iowa Republicans to turn out despite his lead to send a “thundering message” that will resonate through November.

“It is effectively over,” said David Bossie, a Republican National Committee member who oversaw the debates process for the party and was a Trump campaign adviser. “It’s been effectively over since the beginning. This has never been a real race.”

Still, millions of dollars are being plunged into the race by all sides. Mr. Trump’s super PAC recently produced a mailer in New Hampshire that counterintuitively links Ms. Haley to Mr. Trump. The mailer calls her “a BIG supporter of Trump’s MAGA Agenda.” It then tries to attack former Gov. Chris Christie as “an anti-Trump Republican.”

The twist, according to a person working for the super PAC, is that the mailer went exclusively to independent voters in New Hampshire who have voted in Democratic primaries. The idea is that tying Ms. Haley to Mr. Trump will lure those independents to Mr. Christie, which could help the former president stay ahead of Ms. Haley.

It’s just one example of the flurry of tactical maneuvers and advertisements that is now so omnipresent in the early states that one pro-DeSantis ad played on television screens in an Iowa venue on Saturday while Ms. Haley was speaking.

Mr. Trump’s decision to bypass all the debates so far has left his rivals to fight among themselves. On Wednesday, Ms. Haley and Mr. DeSantis are set for their first one-on-one debate, on CNN. Mr. Trump has scheduled an overlapping town hall on Fox News.

Ms. Haley, who has made the case that a Trump nomination will bring too much “chaos,” tried to goad the former president onto the debate stage at a town hall in Indianola, Iowa, urging him to “stop acting like Biden” and stop hiding.

Mr. DeSantis, who has struggled for months to find an effective message that draws a contrast with Mr. Trump, may have landed on one in the waning days: “Donald Trump is running for his issues. Nikki Haley is running for her donors’ issues. I’m running for your issues.”

The Iowa caucuses are quirky. There are no traditional polling places that are open all day. Instead, on a Monday evening of a holiday weekend, more than 1,500 precincts will open in the evening for in-person gatherings that can include speeches and lobbying among neighbors. Temperatures are projected to be in the single digits.

The exercise can advantage the most organized campaigns, and Mr. DeSantis is banking that his super PAC’s much-discussed door-knocking operation will pay late dividends.

“It’s never in our business inevitable,” said Beth Hansen, who managed former Gov. John Kasich’s 2016 Republican run for president. “But we don’t know what it is that is going to change this paradigm. And I don’t think it exists inside the current set of arrows the candidates are using in the quiver.”

Kellen Browning contributed reporting.