Texas Gov. Rick Perry’s late entrance into the race for president — and his quick rise to the top of the Republican field — have caused his main competitors to focus more of their efforts in Iowa to try to slow his ascent here and nationally.

Former Massachusetts governor Mitt Romney, who has spent little time and resources here, has essentially reversed his strategy. Romney’s advisers are scheduling several swings this fall, with the first likely to come within weeks. They are mobilizing a volunteer corps that supported Romney in his 2008 bid and are trying to appeal to voters who did not participate in the caucuses that year.

Rep. Michele Bachmann (Minn.), who remains stronger in Iowa than she does nationally, will campaign almost exclusively in the Hawkeye State between now and the state’s first-in-the-nation caucuses.

The reshaping of the race in Iowa marks a remarkable shift since just last month, when Bachmann had surpassed her rivals with a win at the Ames Straw Poll, Romney was making little effort to campaign here and Perry hadn’t yet joined the race.

The cause of much of the refocus is Perry, whose popularity among evangelical Christians and tea partiers gives him an advantage in a state with a deeply conservative Republican electorate. That could cut off Bachmann from her strongest base of support. And Perry’s momentum from a win here could overpower Romney in subsequent states, such as New Hampshire and Florida, where Romney has invested heavily.

Doug Gross, Romney’s 2008 campaign chairman in Iowa who has remained neutral this year, said Romney’s engagement in Iowa will make the state “ground zero” for the Republican nominating contest nationally. He also said it was inevitable, because Iowa is so important as an early state.

“It rounds out the field, because you’ve got pretty much all segments of the caucus electorate with someone they can support,” Gross said. “It impacts Perry the most. Perry was trying to occupy some space that includes economic conservatives as well as social conservatives. So he’ll have to refocus his efforts. This will be ground zero for the campaign. This will be very hotly contested now.”

Jim Conklin, who heads the Iowa Conservative Union, said Romney can build on his foundation from his 2008 campaign.

“Romney has a very, very strong base here in Iowa, so if he tickles it just a little, they’ll come back to life,” said Conklin, who is supporting former House speaker Newt Gingrich (Ga.) in the race. “I’m sure he’s going to do very well.”

Even though Perry has caused others to adjust their Iowa strategies, his remains unclear. Fresh off a swing through the state last week, the Texas governor has promised to compete everywhere, but he has declared no early state a must-win or more crucial to his path than another.

“Governor Perry is committed to running vigorously in Iowa, New Hampshire, South Carolina and every other state in this country,” spokesman Mark Miner said.

While he was nearly omnipresent in the state four years ago, Romney has campaigned in Iowa only once since starting his 2012 campaign: a trip in August to attend a debate, hold a small-business roundtable and visit the state fair.

The Romney campaign thinks it can appeal to Iowans with a message that speaks to those Republicans who care chiefly about economic issues, as opposed to social issues, and by targeting specific voters.

“We’re going to be in Iowa enough to show that we’re the best candidate to beat President Obama on jobs and the economy,” Romney spokeswoman Andrea Saul said. “We want to win wherever governor Romney’s name is on the ballot.”

Romney’s campaign thinks that while he has a “high floor” of support — he retains a solid coalition of establishment Republicans — he also has a “low ceiling.” The team recognizes that it would be difficult for Romney to win a majority in the caucuses, although he could win by a plurality should several candidates divide the conservative vote.

For Bachmann, her Iowa strategy is largely about money; she spent $1.5 million to win the Ames Straw Poll, according to her campaign, and her faltering support is a reason why her resources are rapidly dwindling.

Bachmann’s advisers say her plan was always to focus on winning in Iowa, where her fiery conservatism and popularity among tea party activists caught on quickly over the summer. What they didn’t necessarily expect was to have too few resources to build campaign organizations in other early states.

“At the end of the day, she has tremendous support in Iowa — it’s her way out,” said Ed Rollins, Bachmann’s senior adviser and former campaign manager. “But she’s not going to be able to raise the $50 million that Romney has.”

Even in Iowa, Bachmann’s operation is much leaner than it once was. Gone are her glitzy bus and blaring sound system. Gone is the advance man with the wire in his ear. In their place recently were a white Ford van and a small handful of staff members directing the candidate into a company that makes grain bins, another that makes solar-powered stoplights and another that butchers cows.

“We’re here doing exactly what we have to do,” Bachmann said outside of Amend Packing Plant in Des Moines, where she walked between giant, dangling sides of beef and promised owners and employees that, as president, she would ease regulations and taxes. “People see the dynamics change, and we’re the ones that are here. We’re meeting with people, talking with people — that’s how it all changes.”