— For five days, the royal-blue bus rumbled through miles of cornfields alongside a popular annual bicycle trek across Iowa. It showed up at a country music concert in Cherokee and at a bacon festival in Ottumwa.

And when the hulking vehicle with thick white block letters that spell “TRUMP” pulled into a Wal-Mart parking lot in Fort Dodge this week, people flocked to it. It didn’t matter that Donald Trump wasn’t inside. The bus alone — with the “Make America Great Again!” slogan extending across its sides — created an irresistible oasis of celebrity politics amid a desert of minivans and shopping carts.

“One hundred people showing up for a staffer? I’ve never seen anything like it,” said Chuck Laudner, a longtime Iowa organizer who oversees Trump’s efforts here. “They kept saying the same thing: They want something different.”

For many Americans, the Trump presidential campaign amounts to a billionaire talking endlessly, and entertainingly, on television. But here in Iowa, it’s another story. Trump is trying to beat the politicians on their turf, building one of the most extensive organizations in the Republican field.

The groundwork laid by Trump’s sizable Iowa staff, with 10 paid operatives and growing, is the clearest sign yet that the unconventional candidate is looking beyond his summer media surge and attempting to win February’s first-in-the-nation caucuses.

How could Donald Trump's campaign possibly still be alive, given all of the impolitic things he's said? The Post's Chris Cillizza has the answer. (Tom LeGro/The Washington Post)

This is becoming a cause of concern for rival campaigns.

“I see them as a major threat to all the other campaigns because of the aggressiveness of their ground game,” said Sam Clovis, an Iowa conservative who leads former Texas governor Rick Perry’s campaign.

“You cannot swing a dead cat in Iowa and not hit a Trump person,” Clovis continued. “It’s unlike anything I’ve ever seen. . . . Every event we go to — the Boone County Eisenhower Social, the Black Hawk County Lincoln ­Dinner, the boots-and-barbecue down in Denison — the Trump people are everywhere with literature and T-shirts and signing people up.

“The Trump bus will pull into an empty parking lot and just be there on the main drag, like the little town of Le Mars, the ‘Ice Cream Capital of the World.’ . . . People will pull over, go sign up. They’ll get 50 people in an hour and go to another town. That happens all over the state.”

Iowa Gov. Terry Branstad (R), in an interview Thursday, said of Trump: “I think he’s got a real campaign here. Whether he’s willing to devote the time to go to as many places as some of the other candidates are going is the question.”

Backers of former congressman Ron Paul’s presidential campaigns, which were well organized in Iowa, see Trump appealing to Paul’s base here despite the competing candidacy of Paul’s son Rand, Kentucky’s junior U.S. senator.

“He’s catching on with the average Americans who have had it with foreign wars, our trade policies and a stalled economy,” said Drew Ivers, Ron Paul’s 2012 Iowa campaign chairman.

Trump’s colorful assault on the political establishment and strident opposition to illegal immigration has propelled his candidacy to the lead here and nationally. A CNN-ORC poll on Wednesday showed him in first place in Iowa with 22 percent, followed by retired neurosurgeon Ben Carson in second with 14 percent. Wisconsin Gov. Scott Walker, who is counting on the caucuses, fell to third at 9 percent.

Trump will make a theatrical return to Iowa on Saturday. He plans to touch down in Des Moines by private helicopter, landing in a field just outside the Iowa State Fairgrounds, and then visit the famed butter cow, according to Republicans familiar with the campaign. He also plans to huddle with activists.

Candidates traditionally give a speech and take questions at the fair’s Des Moines Register Soapbox, but Trump is not planning to do so. He is in a feud with the Register; after the newspaper’s editorial board called on him to withdraw, Trump slammed the newspaper and began barring its reporters from his events.

Other candidates are building solid networks here as well. Former Florida governor Jeb Bush, whose Iowa operation has nine paid staffers, announced campaign chairs in 22 of Iowa’s 99 counties on Wednesday, with more to come. Walker, who has four staffers and two consultants here, unveiled a 65-member Iowa leadership team last week that includes lawmakers, mayors, sheriffs, county treasurers and party stalwarts.

But Trump is taking a different approach. His state director is Laudner, a highly regarded grass-roots tactician and confidant of Rep. Steve King (R-Iowa), who is a powerful force on the hard right. This time four years ago, Laudner drove Rick Santorum around the state in his pickup truck, guiding the former senator from Pennsylvania to a come-from-behind victory in the 2012 caucuses.

“I’ve told people from the beginning: Never underestimate Donald Trump,” said Bob Vander Plaats, president of the Family Leader, an influential social-conservative group here. “He has been very successful for a reason. He knows how to market, and specifically he knows how to market himself, very well. He also understands what the customer wants.”

It is an open question, how­ever, whether Trump’s singular brand of politics will stay in vogue until the February caucuses. And there are doubts that Trump can win enough votes from evangelical Christian conservatives, who are being courted heavily by Sen. Ted Cruz (R-Tex.), former Arkansas governor Mike Huckabee and others.

Last month at the Family Leader’s candidate summit in Ames, Trump, a Presbyterian, caused unease when he said he had never asked God for forgiveness and spoke casually about Holy Communion.

“When I drink my little wine — which is about the only wine I drink — and have my little cracker, I guess that is a form of asking for forgiveness,” Trump said.

Iowa radio host Steve Deace said he would be “very surprised” if Trump won here. “Whatever chance he had to get evangelicals to coalesce around him went out the window at the Family Leader,” he said. “Everyone was paying attention, especially those who are fed up with the Republican Party, but he didn’t sell them.”

Trump is trying to defy conventional wisdom about the caucuses by creating a broad coalition.

It has become a punch line among party insiders that Trump’s Iowa co-chair is Tana Goertz, a political neophyte best known for being a runner-up on Trump’s NBC show “The Apprentice.”

But others on the Trump team are experienced political hands. Co-chair Richard Thornton is a lawyer plugged into state legislative politics, and deputy state director Chris Hupke is a former head of the South Dakota Family Policy Council known for his field organizing. Another top aide is Ryan Keller, who ran congressional campaigns and the Republican Party in Polk County, Iowa’s largest.

Trump has made only occasional campaign stops in Iowa, and he eschews the small retail appearances that other candidates make.

Making up for his absence is the “TRUMP”-emblazoned bus. The campaign advertises on the Web when the bus will be in a town. Residents turn out to get Trump yard signs, Trump pins and Trump T-shirts. More important, they leave their names and contact information and take home kits explaining how to become caucus captains in their precincts, distribute bumper stickers and write letters to the editors of local papers.

Political organizing in Iowa requires sophistication because of the state’s unique system. Voters gather at a designated time with their neighbors and advocate for their preferred candidates before ballots are cast.

Turnout in Iowa caucuses is historically low. In 2012, only 121,000 of the state’s roughly 600,000 registered Republican voters participated. In 2016, strategists expect turnout to increase to 140,000 or higher.

The Trump campaign is targeting voters who may not have participated in a caucus before, modeling its strategy on Barack Obama’s 2008 Iowa campaign, which mobilized tens of thousands of new caucus-goers.

“We’re reaching people that the Republican apparatus doesn’t even know exist,” Laudner said. “The other day, one woman came up to say, ‘Hello, a lifelong Iowan.’ Her first question to us was ‘What’s a caucus?’ After we told her, she wanted to help. . . . Politics has not been the biggest thing in a lot of these people’s lives. They’ve got lots of stuff going on with their jobs or families. But they feel Donald Trump is what this country needs.

Carson also is trying to use his political-outsider status to attract new voters into his camp. The challenge for both candidates will be getting people to show up on a cold night in February.

“It’s these nontraditional candidates, Carson and Trump, who are going out there really trying to bring new people into the process,” said Craig Robinson, editor in chief of the Iowa Republican. “If motivated, sure these people will caucus.”

On a Saturday in late July, Trump swept into Oskaloosa, a town of about 11,000, where he addressed an overflow crowd at the local high school as his bus ­sat parked outside. Wearing a ­salmon-pink tie and dark suit, he gushed about the state.

“Whoa! Beautiful, beautiful,” Trump said. “It’s a terrific place, Iowa. Terrific! We just got in, and I’m driving through these beautiful fields. I want to grab that corn like you’ve never seen. So rich, so beautiful.”

Costa reported from Washington.