DES MOINES, Iowa (AP) — As the six-month sprint to the Iowa caucuses begins, the sprawling field of Republican presidential candidates is facing growing pressure to prove they can become serious challengers to former President Donald Trump.

The urgency is particularly acute for Florida Gov. Ron DeSantis, who entered the race in May with expectations that he would quickly become Trump's top rival. For now, however, he has struggled to generate the level of enthusiasm that Trump commands from the GOP base, contributing to uncertainty that DeSantis will become the threat to the former president that he was once billed to be.

“That’s what DeSantis wanted to be. It’s possible he may be that still,” said Gentry Collins, a seasoned Iowa and national Republican strategist who ran Mitt Romney’s 2008 caucus campaign. “But it sure doesn’t look like that to me — it’s become clear that there isn’t room for another alternative to Trump.”

DeSantis is among six White House hopefuls who will be in Iowa on Friday to appear at the Family Leadership Summit, one of many events that will be held in the state in the coming weeks as voters begin to more seriously consider their options. Trump is not attending, opening him to criticism from some Republicans that he's ignoring the forums that are a staple of Iowa presidential politics.

Trump has swung through the state multiple times in recent weeks and will return Tuesday.

There's still time for any of the contenders to mount a more robust challenge to Trump. But the Iowa Republican Party's recent announcement that the caucuses would take place on Jan. 15 — weeks earlier than the past three open contests — reinforced the reality that candidates aiming for a turnaround are on a timeline.

Beyond DeSantis, Tim Scott is being closely scrutinized. The South Carolina senator has impressed many with an agenda that is every bit as conservative as the one offered by Trump or DeSantis. But some say Scott is distinguishing himself with an aggressive outreach strategy paired with an upbeat message.

“The reason (Scott is) making inroads is he’s doing the real hard work of retail politics in Iowa, doing small groups with pastors and churches and leading to bigger and bigger meetings and venues,” said Mike Demastus, a Des Moines evangelical pastor who has met several times with Scott and sat in on private meetings between other candidates and politically active clergy. “That’s why the needle is moving for him.”

LaTomah Hauff, a retired speech pathologist who lives in Sioux City, is not ready to commit to a candidate. But she is a regular attendee at candidate events in her part of western Iowa and has added Scott to her short list of favorites.

“He’s very passionate about what he believes," she said. "And there is hope and optimism in what he says.”

Still, Trump is the undisputed leader in Iowa, similar to the grip he holds on Republicans nationally. That makes Iowa particularly crucial for anyone hoping to stop the former president. Given the relatively early date of the caucuses next year, a strong win by Trump in Iowa could put him in a commanding position heading into the following contests.

“There’s no question Donald Trump is winning Iowa right now,” said Josie Albrecht, a former top Iowa GOP Statehouse communications adviser who is advising the state party but is neutral in the 2024 campaign. “I think there has been a lot of support for him for many years, and that’s a hard wall to crack.”

Trump is eagerly embracing the lofty expectations. His campaign is bullish on Iowa, banking on his longstanding support in a state he easily carried twice in general elections, combined with an aggressive digital outreach that includes a focus on nontraditional conservative voters.

Yet Trump faces some vulnerabilities, including a feud with Iowa's popular governor, Kim Reynolds, over her refusal to formally endorse his campaign. And while many in the party view recent indictments in New York and Florida as politically motivated, they nonetheless risk becoming a liability that rivals may try to exploit.

In a memo shared last month with donors to the influential network started by Charles and David Koch, Michael Palmer, who leads the group's data and polling operation, argued against what he called “the myth of Trump inevitability.” He wrote that a significant number of Trump voters remain open to a Republican alternative, while citing public polling that indicates DeSantis may be a stronger general election candidate against Biden.

But a central challenge for Republicans is to hone a message that resonates with voters who back have backed Trump, but are open to others in 2024.

Americans for Prosperity, the political arm of the Koch brothers operation, is working to explicitly undermine Trump in Iowa and other early-contest states. Since February, an army of AFP’s paid staff and volunteers has been knocking thousands of doors a week in Iowa raising questions about Trump’s chances in a general election, the group’s state director Drew Klein said.

That approach has concerned some in the GOP. Cedar Rapids Republican Bernie Hayes, chairman of the GOP in Iowa’s second most populous county, said he was shocked when Klein told people last week they shouldn't back Trump in the caucuses.

“Why would you speak against him where there’s a big percentage of people who support Donald Trump?” said Hayes, who is also a member of the state Republican Party central committee and publicly neutral. “That message is going to lose big time.”

Candidates who are the most blunt in knocking Trump aren't making inroads in Iowa. New Jersey Gov. Chris Christie, for instance, hasn't visited the state as a 2024 candidate and is not among the summit speakers Friday. He is instead focusing his energy on the more libertarian voters in New Hampshire.

The super PAC supporting DeSantis, meanwhile, has prioritized promoting the governor without attacking Trump, which advisers say turns off his past supporters.

DeSantis appears to be shifting his early-state strategy to some degree. After largely holding journalists at bay, he's holding what's being described as a press conference Tuesday in South Carolina.

While multiple candidates are hoping for comebacks, DeSantis may ultimately be best positioned for a long slog against Trump. He will almost almost certainly have the resources to stay in the nomination fight long after Iowa Republicans cast their votes. His campaign said he raised $20 million in the first six weeks after his announcement; the super PAC claimed $130 million over the same period.

And while some would-be DeSantis donors have raised concerns, others are moving away from Trump in favor of the governor. They include Tim Michels, the 2022 Republican candidate for governor in neighboring Wisconsin. The construction magnate, who was endorsed by Trump ahead of his midterm loss last fall, attended a DeSantis fundraiser in a Milwaukee suburb Tuesday, according to three people who were at the event but weren't authorized to speak for Michels.

Michels, through a spokesman, didn't return a message for comment.

The fundraiser hosts included Republican mega-donors Dick and Liz Uihlein, who raised money for Trump in Wisconsin and donated to efforts to get him elected in 2016 and 2020. Neither attended the fundraiser, but they were listed on the invitation, among many other locally notable Republicans.

But any recovery for DeSantis will almost certainly be grounded in a strong showing in Iowa. And some in the state say he has the opportunity by continuing to stoke conservative outrage related to rights for transgender people and racial equality.

“People like what they hear from him,” said Demastus, the Des Moines pastor. “He is speaking evangelical love language, protecting our children, pushing back against the woke ideology.”

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Peoples reported from New York. Associated Press writer Scott Bauer in Madison, Wisconsin, contributed.