DES MOINES, Iowa — The accelerating GOP presidential campaigns in Iowa probably will define front-runner Mitt Romney’s chief challengers over the next six weeks and could force the former Massachusetts governor to reconsider his decision to mount only modest efforts in this early voting state.

Rep. Michele Bachmann’s quick rise in popularity in the leadoff caucus state and former Minnesota Gov. Tim Pawlenty’s stubbornly low poll numbers after more than a year of groundwork in Iowa give Romney new opportunities in the state where he has worked to lower expectations in his second campaign.

Romney may stick with his plan to tread lightly in Iowa and look to New Hampshire’s leadoff primary for a liftoff in 2012 if there is no opening for him to seize as a consensus choice.

But Romney’s healthy fundraising, with as much as $20 million in the three-month reporting period that ended last week, and his lead in national polls give him flexibility his less-known rivals lack and make it possible for him to wait to see how the chips fall in Iowa this summer, and decide later whether to up his ante.

“I think it’s awfully hard for me at this stage to predict where we’ll spend all our time and devote all our resources,” Romney told The Associated Press this past week. “But we’re focused on running our race, where we think best.”

Minnesota’s Bachmann was on her first sustained Iowa campaign trip this weekend. She’s coming off a successful stretch marked by a well-received national debate debut, a widely covered campaign kickoff in her native Iowa and a strong showing in The Des Moines Register’s poll. Bachmann nearly matched Romney, the No. 2 GOP caucus finisher four years ago, for the early Iowa lead in the survey.

Criticized for having little caucus campaign heft on her team, Bachmann has named as her deputy national campaign manager David Polyansky, who’s credited with bringing organizational and strategic weight to the 2008 campaign of caucus winner Mike Huckabee.

Bachmann has feet in Christian conservative and tea party camps, and will need to quickly organize within these groups. Polyansky, who helped Huckabee form relationships with Christian home-school advocates in Iowa, for instance, can help behind the scenes. Bachmann’s schedule had her headlining a tea party rally in Des Moines on Saturday.

But caucus support is more often sealed in person than in crowds at a rally or along a July Fourth parade route. Bachmann will have to meet privately with influential GOP activists, as she plans to begin this weekend. She’s also staffing a phone bank to drum up support for the Aug. 13 straw poll. “We can’t make enough personal appearances in 40 days to make that happen,” said Bachmann’s Iowa campaign chairman, Kent Sorenson.

Businessman Herman Cain, a tea party favorite, also will need strong support from this motivated but untested segment of the GOP electorate. Cain, third in the new Iowa poll, was the only other candidate in double digits, with 10 percent. But his campaign organization has suffered some key staffing departures in Iowa.

Former Sen. Rick Santorum of Pennsylvania has a foothold among social conservatives. He’s looking for a straw-poll breakthrough with help from a top aide to Romney’s 2008 campaign.

For Pawlenty, the task in Iowa is the opposite of Bachmann’s. He will spend 15 days in the state this month trying to show that the organization he has built there can generate enthusiasm.

Pawlenty has the largest Iowa campaign staff, has spent more than two dozen days in the state since November 2009 and is airing the campaign’s first television ads. He has a list of recognizable Republican supporters, from former statewide officeholders to up-and-coming figures. Yet he was the choice of only 6 percent in the Register’s poll.

After saying in January he needed to “win or do very well” in the caucuses, he recently has tried to lower expectations for the straw poll, despite hiring the consultant who helped Romney win the 2007 straw poll.

“As to the straw poll, I don’t know that we need to win it,” Pawlenty told a conservative radio host in Des Moines this past week. “I think we need to do well and show some progress.”

The pressure is on Pawlenty to assemble those pieces of the broad GOP coalition he has long said he can deliver, including social and business conservatives.

“Tim Pawlenty is coming up on a pretty serious EKG test in Iowa,” said Robert Haus, who ran the 2008 Iowa caucus campaign for former Sen. Fred Thompson of Tennessee. “Is he going to ultimately materialize into the big challenger to Romney or not?”

Pawlenty’s lack of early momentum could open the door for Texas Gov. Rick Perry, who could attract support from Iowans looking for a pro-business governor besides Romney. Perry, who’s also popular with social conservatives, is considering a White House run and plans a national day of prayer in Houston for Aug. 6, a week before the straw poll.

A top Perry adviser, Dave Carney, has made inquiries in Iowa about the timing and rules of the straw poll and caucuses, while Perry has raised his profile with key appearances and private meetings with influential Republicans.

Perry will claim the space in Iowa for a pro-business governor if Romney does not, said Doug Gross, a top Iowa backer of Romney’s 2008 campaign who tried to coax Indiana Gov. Mitch Daniels into running.

“Unless Romney gets in and campaigns here, he will only go down, leaving an opportunity for a Perry,” Gross said.