Over decades of presidential campaigns, the Iowa way has been to hop from town to town, taking questions from all comers and genuflecting to the local culinary traditions. Going everywhere and meeting everyone has been the gospel of how to win over voters in the low-turnout midwinter caucuses that kick off the American presidential cycle.

Now former President Donald J. Trump is delivering what could be a death blow to the old way.

Five months from the 2024 caucuses, Mr. Trump holds a comfortable polling lead in a state he has rarely set foot in. If any of his dozen challengers hope to stop his march to a third straight nomination, they will almost certainly have to halt, or at least slow, him in Iowa after spending the better part of a year making their case. A commanding victory by Mr. Trump could create a sense of inevitability around his candidacy that would be difficult to overcome.

As Mr. Trump and nearly all of his Republican rivals converge in the coming days at the Iowa State Fair, the annual celebration of agriculture and stick-borne fried food will serve as the latest stage for a nationalized campaign in which the former president and his three indictments have left the rest of the field starved for attention.

“You’ve got to do it in Iowa, otherwise it’s gone, it’s all national media,” said Doug Gross, a Republican strategist who was the party’s nominee for governor of the state in 2002. “The chance to show that he’s vulnerable is gone. You’ve got to do it here, and you’ve got to do it now.”

Most of the Republican candidates are trying to do Iowa the old way, and all of them are less popular and receiving far less visibility than Mr. Trump, who has visited the state just six times since announcing his campaign in November.

The same polling that shows Mr. Trump with a wide lead nationally and in Iowa also indicates that his competitors have a plausible path to carve into his support in the crucial first state. A recent New York Times/Siena College poll found that while Mr. Trump held 44 percent of the support among Iowa Republicans — more than double that of his closest rival, Gov. Ron DeSantis of Florida — 47 percent of Mr. Trump’s supporters said they would consider backing another candidate.

Mr. DeSantis, for all his bad headlines about staff shake-ups, campaign resets and financial troubles, holds significant structural advantages in Iowa.

He has endorsements from a flotilla of Iowa state legislators; a campaign team flush with veterans from the 2016 presidential bid of Senator Ted Cruz of Texas, who beat Mr. Trump in the state; and a super PAC with $100 million to spend. Mr. DeSantis has also said he will visit all 99 counties, a quest that has long revealed a candidate’s willingness to do the grunt work of traveling to Iowa’s sparsely populated rural corners to scrounge for every last vote.

Convincing Iowans that they should be searching for a Trump alternative may be Mr. DeSantis’s toughest task.

“Trump’s supporters are very vocal, so sometimes being very vocal sounds like there’s a lot of them,” said Tom Shipley, a state senator from southwest Iowa who has endorsed Mr. DeSantis. “That doesn’t necessarily mean that’s the case.”

Yet while Mr. DeSantis has drawn receptive crowds and has been cheered at the state’s big political events, there is no flood of Iowans rushing to support him. Through the end of June, just 17 Iowans had given his campaign $200 or more, according to a report filed to the Federal Election Commission. Nikki Haley, who lags far behind him in polls, had 25 such Iowa donors, while Mr. Trump had 117. Former Vice President Mike Pence had just seven.

(The number of small donors Mr. DeSantis had in Iowa is not publicly known because his campaign has an arrangement with WinRed, the Republican donor platform, that effectively prevented the disclosure of information about small donors.)

Mr. DeSantis’s supporters are quick to point out that the three most recent winners of competitive Iowa caucuses — Mr. Cruz, Rick Santorum in 2012 and Mike Huckabee in 2008 — each came from behind with support from the same demographic: social conservatives. None of the three won the presidential nomination, but all of them used Iowa to propel themselves into what became a one-on-one matchup with the party’s eventual nominee.

Operatives and supporters of the non-Trump candidates warn that Iowa caucusgoers are notoriously fickle. Around this point in 2015, Mr. Cruz had just 8 percent support in a poll by The Des Moines Register. Mr. Trump was first at 23 percent and Ben Carson was second, with 18 percent.

“It’s a marathon, not a sprint,” said Chris Cournoyer, a Republican state senator from Le Claire who is backing Nikki Haley, who was at 4 percent in the recent Times/Siena poll.

What’s different about Iowa this time, according to interviews with more than a dozen state legislators, political operatives and veterans of past caucuses, is that before Republicans consider a broad field of candidates, they are asking themselves a more basic, binary question: Trump or not Trump?

Where in the past Iowans might have told those running for president that they were on a list of three or four top contenders, Mr. Trump’s dominance over Republican politics has left candidates fighting for a far smaller slice of voters. The longer a large field exists, the harder it will be for Mr. DeSantis or anyone else to consolidate enough support to present a challenge to Mr. Trump.

“These people are absolutely going to vote for the former president, and those people are absolutely not going to vote for the former president,” said Eric Woolson, who has been in Iowa politics so long he was part of Joseph R. Biden Jr.’s 1988 presidential campaign before working for a series of Republican presidential hopefuls: George W. Bush, Mr. Huckabee, Michele Bachmann and Scott Walker.

Now Mr. Woolson, who owns an organic catnip farm in southern Iowa, serves as the state director for Gov. Doug Burgum of North Dakota, who is polling at 1 percent in Iowa. Mr. Woolson said the first hurdle for 2024 campaigns was sorting out which voters would even consider candidates other than Mr. Trump.

“In past elections, voters were keeping an open mind of, ‘Well, maybe I can still vote for this candidate, or maybe this one’s my second choice or whatever,’” he said. “Now there’s just such stark lines that have been drawn.”

Those lines are compounded by a political and media environment centered not on Iowa’s local news outlets but on conservative cable and internet shows.

For decades, presidential candidates from both parties have flocked to The Des Moines Register’s state fair soapbox, a centrally located stage that has served as a gathering spot for the political news media and passers-by on their way to the Ferris wheel and the butter cow. It was at the soapbox in 2011 where Mitt Romney responded to a heckler with his infamous quip, “Corporations are people, my friend.”

Mr. Trump skipped The Register’s soapbox in 2016 in favor of a far more dramatic appearance — landing at the fair in his helicopter and offering rides to children.

This year, only lower-polling candidates — Ms. Haley, Mr. Pence and Vivek Ramaswamy, among others — are scheduled to speak at the soap box. All of the contenders except Mr. Trump will instead sit for interviews at the fairgrounds with Gov. Kim Reynolds of Iowa, a Republican who has pledged to stay neutral but has clashed with Mr. Trump. The scripted nature of those appearances is likely to cut down on the kinds of viral moments that once drove politics at the fair.

Mr. Trump does not need to participate in Iowa’s retail politics, his supporters say, because he is already universally known and has been omnipresent on the conservative media airwaves as he fights against his indictments.

“Trump can rely on the network that’s out here already,” said Stan Gustafson, a Republican state representative from just south of Des Moines. “It’s already put together.”

Yet at least a few Iowa Republicans supporting Mr. Trump say they are looking to the future — just a bit further out than next year’s caucuses. Mr. Gustafson, who has endorsed Mr. Trump, said he was eyeing which candidates he might support in 2028.

Tim Kraayenbrink, a state senator who also backs Mr. Trump, said Iowa’s turn in the campaign cycle was a good opportunity to judge which candidates would make a good running mate — as long as it is not Mr. Pence, he clarified.

“He’s going to have some quality people to choose from for vice president,” Mr. Kraayenbrink said of Mr. Trump.

Andrew Fischer and Alyce McFadden contributed reporting.