The 2024 Republican presidential race is entering a fraught and caustic new phase, as Donald J. Trump’s wide lead remains undiminished, the days until voting begins dwindle and his rivals take aim at each other as much as at him.

Ahead of Wednesday’s debate in Miami, the campaigns of Ron DeSantis, Nikki Haley and Tim Scott each put out memos on the state of the race that knifed each other’s viability, skills and standing, in a bid to present themselves as the only true alternative to Mr. Trump, the man who refuses to debate them. The Trump campaign put out a memo, too. It ignored his primary rivals entirely, and instead previewed his run against President Biden, one year out from the general election.

The dueling memos and drastically different schedules in the run-up to the debate — Mr. Trump was in court giving sworn testimony in his financial fraud case, while his rivals were readying their debate zingers and campaigning in Iowa — captured the stark reality of a primary that is proceeding on two parallel tracks.

There is Mr. Trump, the front-runner. And there are his Republican opponents, increasingly doing battle with an unseen force that threatens them all in equal measure: a growing sense of inevitability and resignation — among donors, Trump skeptics and Democrats alike — that 2024 will be a rematch of 2020.

The primary is obviously not over, despite the Trump team’s attempt to brand it as the race for “first place loser.” Polls often shift late. No votes have been cast.

Yet Mr. Trump’s fractured opposition, and the persistent focus on one of them emerging as the leading “Trump alternative,” echo the dynamics of his first run in 2016, when his rivals spent millions of dollars on ads attacking each other while he marched to the nomination.

“At least that was a viable strategy then,” said Sarah Isgur, who was a top adviser to Carly Fiorina’s presidential campaign that year. “Because at least if you knocked out everyone, you could have beaten Trump. That’s not true this time. Even if you got a one-on-one race, I don’t see the math.”

Liam Donovan, a Republican strategist, said Mr. Trump’s rivals appeared to be mindlessly repeating the mistakes of the past. “Despite what has amounted to a rerun, Trump’s challengers seem determined not to try anything new at all,” he said.

Besides Mr. Trump, only one of the 2024 candidates ran in 2016: former Gov. Chris Christie of New Jersey. And he has sounded the alarm on the strategy of focusing more on each other than Mr. Trump. In a recent slide presentation prepared for donors, his campaign faulted what it called “wishful thinking by the other candidates that Trump will magically collapse on his own,” though Mr. Christie’s broadsides have so far not significantly advanced the candidate himself.

For now, Iowa, increasingly, is the epicenter of the action.

In a blunt admission, the DeSantis campaign said in its memo that a blowout Trump victory in Iowa could effectively give him the nomination. “If Trump were to win big in Iowa it would create media and political momentum for his candidacy that would be difficult to stop,” wrote James Uthmeier, Mr. DeSantis’s campaign manager.

For months, Mr. DeSantis’s aides have said they are best positioned to stop Mr. Trump, and on Monday, he formally secured the coveted endorsement of the state’s popular Republican governor, Kim Reynolds. She pledged to do everything she could to lift Mr. DeSantis to victory in the first-in-the-nation contest, after his campaign has spent months in a downward trajectory.

Mr. Trump’s team countered with the endorsement of another female Republican governor, Sarah Huckabee Sanders of Arkansas. On Wednesday night, Ms. Sanders will appear with Mr. Trump at a rally near Miami, the scene of the former president’s counterprogramming to the nearby Republican primary debate.

But Iowa is where Mr. Trump’s allies continue to pound Mr. DeSantis with attack ads. It is where Mr. DeSantis’s super PAC is thrashing Ms. Haley on the airwaves, and where her super PAC is hitting him back. It is where Mr. Scott is stationing his campaign staff. And it is where Vivek Ramaswamy’s campaign has announced plans to spend the bulk of a late $10 million blitz in the next two months.

Each of the campaigns — but especially Mr. DeSantis’s and Ms. Haley’s — are fighting a two-front war in Iowa: both to close the gap with Mr. Trump and to distance themselves from each other and the rest of the pack. A Des Moines Register/NBC News/Mediacom poll showed Mr. Trump with 43 percent support, and Ms. Haley and Mr. DeSantis tied at 16 percent.

Ms. Haley’s team has outlined a different path beyond Iowa. They said she is more competitive in New Hampshire and South Carolina, her home state, than Mr. DeSantis. Betsy Ankeny, Ms. Haley’s campaign manager, wrote in her memo that Mr. DeSantis was a “sinking ship” who had worn poorly on voters after pundits first predicted he would be Mr. Trump’s “strongest alternative.”

“And then,” she wrote, “America met Ron DeSantis.”

Mr. Scott, whose super PAC canceled its television ads and is polling in the single digits, is attempting to exploit the battle between Ms. Haley and Mr. DeSantis in Iowa. “She’ll attack him for the failing candidate that he is,” read a memo from Mr. Scott’s campaign manager, Jennifer DeCasper. “He’ll attack her for being the moderate that she is. They’ll both be right.”

Mr. Ramaswamy’s campaign has faded since the first debate, but he recently announced a significant advertising buy. His advisers hope those ads, along with a strong debate performance, will inject his name back into the headlines.

Mr. Christie is alone in skipping Iowa and staking his candidacy on a strong performance in New Hampshire, in the hopes of being the last candidate standing against Mr. Trump by the time Super Tuesday rolls around in early March.

Yet he and Ms. Haley will be competing for similar voters there, particularly if she has a better-than-expected finish in Iowa.

And in South Carolina, Mr. Trump continues to dominate as the two home-state candidates, Ms. Haley, the former governor, and Mr. Scott, the state’s junior senator, trail far behind.

A set of New York Times/Siena College polls over the weekend showed Mr. Trump ahead of Mr. Biden in five of the six most important battleground states, the latest data point to undermine the argument made by some of Mr. Trump’s challengers that he cannot win in 2024.

John Kasich, the former governor of Ohio who was one of the last alternatives to Mr. Trump’s standing in the 2016 primaries, underscored the notion that things can still change. “The one thing we know about politics is what makes it interesting — what’s certain today is not certain tomorrow,” Mr. Kasich said. “We live in a time of black swan events.”

The Trump team has barely attempted to hide its glee at the infighting as it focuses entirely on undermining Mr. DeSantis, attention that the governor’s team believes proves their point about Mr. DeSantis’s role as the only serious threat.

Mr. Trump has spent much of his time casting himself as a victim of Democratic and prosecutorial overreach, and galvanizing his base with talk of “election interference” and “persecution.”

There is little to distinguish between Mr. Trump’s trials and his campaign these days. He previews his court appearances in fund-raising texts. He recaps them in fund-raising emails. On the stand, the judge in the case on Monday, Arthur F. Engoron, chided Mr. Trump: “This is not a political rally.” Outside the courthouse, as one of his lawyers, Alina Habba, addressed the media, two of his political advisers, Steven Cheung and Jason Miller, could be seen in the background.

Yet his rivals — apart from Mr. Christie — rarely mention his legal jeopardy or his four criminal indictments. In an interview on MSNBC last week, Mr. DeSantis said a conviction in Mr. Trump’s criminal trials would be fatal, adding, “I don’t think the party should nominate in that situation.”

Mr. DeSantis had shunned MSNBC until his campaign found it needed more attention. He and others have discovered, just as in 2016, attention is scarce for Mr. Trump’s challengers.

Mr. Trump’s testimony on Monday garnered wall-to-wall coverage on cable news, even as Mr. DeSantis scored what was his most valuable endorsement of the cycle with the backing of Ms. Reynolds, who is bucking tradition in order to boost him. In a bid to draw attention to himself and goad Mr. Trump onto the debate stage, Mr. DeSantis mocked the former president’s masculinity, using a schoolyard taunt in a Newsmax interview. He was responding to the Trump campaign circulating stories about Mr. DeSantis’s boots, and whether they included height-boosting lifts.

The contretemps stirred flashbacks for Republican operatives who had worked on the 2016 race during the waning days of Senator Marco Rubio’s campaign, when he briefly decided his path to victory and attention was to mock the size of Mr. Trump’s hands suggestively. He soon dropped out after losing in his home state.

Mr. Trump’s campaign has for months deployed its own brand of abuse and mockery of Mr. DeSantis. But voters seem to hold him to a different standard; two days after Mr. DeSantis made his crude taunt to Mr. Trump, he did not repeat any version of it at a speech on his home turf, a Republican Party of Florida gathering outside Orlando.

The candidate who delivered the most stridently anti-Trump message there, Mr. Christie, was booed loudly.