The three Republican presidential candidates who are leading in polls in Iowa have settled on three very different theories about what it will take to do well in the caucuses.

Donald Trump and Sen. Marco Rubio (Fla.) have wagered that the state that prides itself on its quirky political traditions is becoming more like the rest of the nation. Stumping only at mega-rallies, Trump might be thought of as having a “field of dreams” campaign: If he builds it, the voters will come — drawn to his message, bombast and celebrity.

Rubio seems to believe Iowa can be won with an air war — much like a primary campaign, but in a state more accustomed to getting to know the candidates in person. In the final three weeks, 1 in 3 TV ads currently booked is for Rubio, paid for either by his campaign or by a super PAC, according to the Des Moines Register. But even Rubio’s supporters are complaining that he has not gotten out enough in the state.

Sen. Ted Cruz (Tex.), the narrow front-runner in recent polls, is taking a traditional approach, albeit one supercharged by technology. Not only is he going out and putting in the effort across the state, but he also is the first in a long line of social-conservative-friendly candidates with the money to build sophisticated ­voter-targeting programs.

“It’s not clear to me that everyone who shows up at a rally for Donald necessarily will show up at Election Day and vote for him,” Cruz said in an interview on his campaign bus. “They may well or they may not. I don’t know the answer to that.”

GOP presidential candidate Sen. Marco Rubio speaks in Cedar Rapids, Iowa. Rubio is increasingly painting immigration as a national security issue rather that a question of what to do with people who are in the country illegally. (Patrick Semansky/AP)

The bus was chugging between Oelwein (population 6,261) and New Hampton (population 3,568) on a grueling six-day swing in which Cruz made 28 campaign stops, some at towns so small that locals said they had never seen a presidential candidate in the flesh.

“I believe that the grass-roots support earned one day at a time and one vote at a time is far more durable and has far greater passion than support that is a result of a momentary political sizzle,” Cruz said.

Having locked into their strategies, the candidates now await the voters to settle on the choices they will make Feb. 1.

“In Iowa, things change very late,” said Ann Selzer, director of the Des Moines Register-Bloomberg Politics poll. “In our final polls before previous caucuses, we have seen the lead shift during the four days we are interviewing in the field. The candidates want their supporters to lock in, obviously. But is there an upside for caucus-goers?”

A Des Moines Register survey published Wednesday shows Cruz three points ahead of Trump, 25 percent to 22 percent, while Rubio continues to run a distant third, with 12 percent. The poll also showed that a majority of likely caucus-goers — 56 percent — could change their minds.

The voters’ quandary was on display every day last week, as the three candidates tore across the state, sometimes close enough for voters to take them all in. Leann Rasmusson, who works part-time at Opportunity Village, a job-training center for people with disabilities, attended Cruz’s event Friday morning at a church in Mason City. Days earlier, she had seen Rubio at a hotel just across the town square. On Saturday, she was driving to nearby Clear Lake to see Trump.

“Who can beat Hillary Clinton?” she said, referring to the Democratic front-runner, as she waited to hear Cruz’s pitch. “Who can stand up to her? I hate to say it — they’re going to have to be just as ruthless as she is.”

Rasmusson worried that Trump might “lose interest” in the presidency, and she was not impressed when Rubio “pretty much went through his speech I’ve heard on TV 100 times.” She caucused for former Arkansas governor Mike Huckabee in 2008. But in 2016, Cruz’s campaign and that of former Florida governor Jeb Bush have been contacting her the most.

At events across the state, some of the most active conservative voters had similar stories. Rubio, who for months had the highest favorable ratings of any candidate, had slipped a bit. To the amusement of conservatives, an ad barrage by the pro-Bush super PAC Right to Rise had focused on Rubio, who is polling about half as strong as Trump and Cruz.

Jordan Russell, Rubio’s Iowa spokesman, said the candidate’s organization in the state is deeper than some may realize.

“Look at the agricultural leaders list and the business leaders list we’ve released,” he said. “By the end of the week, we’ll have leadership in all 99 counties, over 300 people, and that’s going to demonstrate a broad breadth of support. They didn’t come on board yesterday — they’ve been for us for a long time. It’s pretty obvious we’re consolidating a strong portion of the vote here.”

One question, unanswerable for three more weeks, is whether Trump can win here based on tangible excitement and invisible organization. Rep. Steve King (R), a popular northwest Iowa conservative who has endorsed Cruz, called it an “open question whether Trump somehow energizes people that are not connected,” and he said that his political network detected little “ground game” for Trump.

Cruz’s campaign is allowing itself no room for error. On Monday afternoon, 20 volunteers, a number whom arrived from Texas, were working the phones for Cruz at an office park in the Des Moines suburb of Urbandale. A sign on the wall set their goal at 12,000 calls a day. When they exceed it, deputy director Spence Rogers, an Army veteran, does push-ups. Bryan English, Cruz’s Iowa director, said an additional 20 to 25 volunteers were knocking on doors — as they had been over the weekend, when the temperatures were about zero.

At night, after people get off work, as many as 60 people may be working the phones at Cruz’s campaign office, English said. Signs remind volunteers to “Please stay on script!” and to “Avoid the temptation to comment on other candidates,” in part because the campaign is focusing on turning out voters who pledged to support Cruz.

“The model for grass-roots campaigning in the Iowa caucuses goes back a long way,” English said. “You look at what Obama did in 2008, with a level of technology being deployed that nobody had seen before. In the Cruz campaign, we’ve got a marriage of those two components that maybe is unique.”

English was skeptical of how much Rubio will gain by dominating the airwaves in the final stretch to Feb. 1. “I would have said going into this that television advertising is becoming less important,” because people get so much more of their information from the Internet and social media, he said.

A visit later that day to Trump’s headquarters found a smaller and less well-oiled machine. Amid pizza boxes and a few mismatched couches, only five people were working the phones in a shabby strip mall office in West Des Moines. Some were reminding voters that they had to be registered Republicans by caucus night.

In the Trump campaign’s calculus, all was going as planned. Other campaigns would classify a voter as a “0” or a “1,” because he or she might vote in general elections, but not caucuses. The Trump campaign considers that person gettable, as a way to expand the electorate.

If the caucus night turnout cracks the record and reaches 140,000, Trump will have a good night, longtime Iowa operatives say. If the turnout soars to 150,000, as some think is possible, it is likely to be a great night for him.

The path to that record cuts through every town in Iowa. Steve Reindl and Steve Leake were at Casey’s General Store in Manly (population 1,313) when Cruz’s bus dropped by Friday. Neither could remember a presidential candidate stopping in Manly, although word in town had it that Clinton and her entourage had recently passed by on the interstate.

Reindl runs a Goodyear store, and Leake owns Buck’s Recycling, a family-run scrap metal operation. Both say their businesses have been hurt by U.S. trade policy with China.

The men are committed to supporting Trump — and they fit the profile of the new participants the candidate thinks he can bring out Feb. 1.

Neither had caucused before. But both have voted consistently Republican in general elections. And both said they will be at their local caucuses this year. Leake even went to the courthouse to make sure he is registered as a Republican, and to find out where his caucus spot is.

“I probably ought to check into that,” Reindl said.

Neither has had any direct contact with Trump’s campaign, although both have been following it closely through the media. That gave them a sense, more concrete than any “ground game” talk, that Trump’s strategy is working.

“It’s the most important election in my lifetime, because this country is going to hell,” Leake said.

“I’m in the same boat,” Reindl added. “I think there’s a movement this time.”

The following day, both drove 20 miles to Clear Lake and waited half an hour in near-zero temperatures to get into Trump’s rally, then stood for another hour and a half to see the man himself.

Trump did not disappoint, Leake said. “I don’t disagree with almost anything he said.”

James Hohmann and Jose A. DelReal contributed to this report.