When Nikki Haley dropped out of the 2024 presidential race in early March, she withheld endorsing Donald J. Trump and extended a pointed invitation for him to court her and the political coalition she constructed. “This is now his time for choosing,” she said then.

It has been nearly three weeks. He has not called.

There has never been very much magnanimity in the MAGA movement.

But as Mr. Trump prepares for a rematch against President Biden that is expected to offer little margin for error — the last race was decided by fewer than 50,000 votes across three states — the question is whether Mr. Trump’s decision to bypass any sort of reconciliation with Ms. Haley after a brutal and personal primary will matter.

Even out of the race, Ms. Haley has continued to pull in a significant number of voters in ongoing primary contests. Across the five swing states that have held primaries so far — Arizona, Georgia, Michigan, North Carolina and Nevada — a total of about 750,000 people cast ballots for Ms. Haley.

Those five states were decided by a combined roughly 250,000 votes in the 2020 general election.

Some of Ms. Haley’s votes have come from supporters who cast their ballots before she exited the race. But the votes she has drawn are from some of the most critical demographic and geographic swing blocs in the nation.

“I think I speak for many people in that we’re extremely frustrated that we have Biden and Trump to pick from,” said Irma Fralic, a co-chair of Women for Nikki who lives in the swing state of Pennsylvania and who is undecided about whom to vote for in November. “The risk for Donald Trump would be that people would not vote because they’re not happy.”

By some measures, roughly half of Haley voters have said they will back Mr. Biden in November. In North Carolina, exit polling showed roughly four in five Haley supporters would not commit to voting for the Republican nominee. What is less clear is if these were anti-Trump Democrats casting ballots in the Republican primary or dissident Republicans whom the party will need to bring back into the fold.

While Mr. Trump has not reached out to Ms. Haley personally, he has reflected some of the same concerns she aired about her party’s handling of abortion. Mr. Trump’s caution on the issue has come even though he has bragged about appointing the judges that led to the end of federal abortion rights.

The Biden team sees an opening to exploit, and it has begun testing the effectiveness of different messages to determine how to sway disenchanted Haley supporters to their side, according to two people familiar with the matter.

But bypassing niceties after crushing Republican rivals is one of many political norms Mr. Trump has exploded, practicing his distinctive “they always bend the knee” brand of politics that for years has forced his rivals to either heel or be pushed toward the exits entirely.

Even his own vice president’s recent rejection merited little more than a dismissive wave. “I couldn’t care less,” Mr. Trump said.

Mr. Trump’s advisers have said his speedy march to the nomination by landslide margins has left the party unusually unified this early after a contested primary, and they are not treating Ms. Haley as a leader with a loyal following in need of courtship.

“Our doors are always open for anyone who wants to join Donald Trump,” said Chris LaCivita, a top Trump adviser. “Our outreach is based off of the contrasts between President Trump and Joe Biden, and on the issues.”

Dave Carney, a Republican strategist in New Hampshire, said the idea that securing Ms. Haley’s support would unlock her coalition was an outdated way of looking at American politics.

“In the olden days,” Mr. Carney said, “you would cut a deal — you would be secretary of state and you campaign for me and bring all your people. Those days are long gone.”

Now, American politics have become increasingly tribal, and the pressures for even dissatisfied Republicans not to oppose Mr. Trump in November are significant.

“They’re not Haley voters,” Mr. Carney said of Ms. Haley’s primary voters. “They’re voters who voted for Haley.”

Those same partisan dynamics are among the reasons the Biden team has remained confident that the parts of his base who have flirted with Mr. Trump — Black, Latino and young voters — will eventually return to the fold.

Still, at times, the Trump operation has seemed to actively push Ms. Haley’s supporters away. In January, Mr. Trump threatened that donors who gave to Ms. Haley would be “permanently barred from the MAGA camp.” The result was one of Ms. Haley’s strongest fund-raising days.

The morning after Super Tuesday, Mr. LaCivita, a decorated combat veteran, posted an image from a famous scene in “Apocalypse Now,” when Robert Duvall’s character declares, “I love the smell of napalm in the morning.” It communicated perhaps more than Mr. LaCivita intended.

Here was a landscape burned, flattened and wasted after a war. It was a moment that recalled the brutal and deeply personal ordnance Team Trump first used against Gov. Ron DeSantis.

Two hours later, Ms. Haley formally conceded in South Carolina, declaring that Mr. Trump has “to earn the votes of those in our party and beyond it who did not support him.” The exact minute she congratulated him — 10:05 a.m. — Mr. Trump posted on his social media site that Ms. Haley had been “trounced,” and goaded her to stay in the race to suffer further defeats. His campaign later circulated a statement claiming that post had come “before” her speech.

The Biden campaign, in contrast, issued a statement from the president welcoming Haley supporters. “There is a place for them in my campaign,” Mr. Biden said.

Mr. DeSantis endorsed Mr. Trump the day he quit the race in January. But after a leak in February of private remarks by the governor that were somewhat critical of Mr. Trump, the Trump team quickly snapped back into battle mode, mocking his weight and calling him a “sad little man.”

Mr. Trump’s handling of the early stages of his nomination stands in stark contrast to the last competitive Democratic primary. In 2020, Mr. Biden’s team created joint task forces on policy with his last rival, Senator Bernie Sanders. Progressives said the resulting good will was significant.

“Immediately what it said is there is an actual attempt to bring people together,” recalled Ari Rabin-Havt, Mr. Sanders’s deputy campaign manager.

Now, senior Biden officials said they are in near daily conversations with Ms. Haley’s financial supporters.

Jeffrey Katzenberg, the Hollywood mogul who is a co-chair of the Biden campaign, said Mr. Trump was “chasing people away from his own party.” He added that he viewed his job as working to “put the red carpet out and welcome them in.”

For years, Mr. Trump has added to his strength inside the Republican Party by subtracting rivals. He has regularly slashed at Senator Mitt Romney, the party’s last nominee before him, as well as Paul Ryan, the former House speaker, and Karl Rove, the architect of George W. Bush’s presidential campaigns.

Many are dubious that Ms. Haley won’t eventually fall in line, along with most of her supporters.

“He’s not treating her any better or worse than he does on the whole any other people,” said Philippe Reines, a former senior adviser to Hillary Clinton. “And if you look at those other people, they come around. Look at Ted Cruz. He calls his wife ugly, accuses his father of killing J.F.K. and you couldn’t find a guy who’s more in the tank.”

The list of those who came to back Mr. Trump after talking tough is long, including most recently Mr. DeSantis. The former Gov. Rick Perry of Texas called Mr. Trump a “cancer on conservatism” in 2015 before serving in his cabinet. Senator Mitch McConnell of Kentucky, the Republican leader who said Mr. Trump’s actions around the Jan. 6, 2021, riot were a “disgraceful dereliction of duty,” endorsed him this month.

In the Republican primaries, Mr. Trump has performed weakest — and Ms. Haley has performed best — in the suburbs. Those are the same areas that have contributed to Republican defeats since Mr. Trump’s 2016 election.

In Johnson County, the largest county by population in Kansas, which encompasses the suburbs of Kansas City, Mr. Trump won only two-thirds of the vote, with Ms. Haley pulling in 22 percent and a “none” option garnering seven percent. And in the swing state of Arizona, Ms. Haley ran strongest in Maricopa County — which encompasses Phoenix and its suburbs — pulling in 21 percent.

Major pro-Trump voices are writing off those votes already.

“Screw Nikki Haley — we don’t need her endorsement,” Stephen K. Bannon, the influential former Trump adviser, said on his podcast after Super Tuesday. He mimicked the critics calling for a “Kumbaya” moment, adding, “‘Oh, we’ve got to get the Nikki Haley voters’ — they’re all Democrats. They’re all going to vote for Biden anyway.”

John R. Kasich, the former governor of Ohio who was one of the last candidates standing against Mr. Trump in the Republican primary in 2016, said most Republicans would support Mr. Trump because of how divided the nation had become, even if the former president’s behavior “doesn’t help him.”

“We’re just so polarized that they might say, ‘I don’t like Trump, but I’m not going to vote for Biden,’” Mr. Kasich said. After eight years, he said, most had grown accustomed to how Mr. Trump acts.

“If they do pay attention, they go, ‘Oh, that’s just Trump,’” he said. “‘That’s just Trump.’”

Jonathan Swan contributed reporting.