LE MARS, IOWA — It will take money to win the Republican presidential straw poll this summer in Ames, Iowa, and Mitt Romney is unabashed about using his to buy votes.

Campaigning at an ice cream parlor in this town 200 miles away last week, the former governor of Massachusetts offered free bus rides to "anybody that wants to come out to Ames and vote for me."

"If you want to vote for somebody else, you might buy yourself an RV," he joked to a crowd of Republicans sipping milkshakes.

Romney's spending on the bus rides is part of his multimillion-dollar push to vault himself to the front of the Republican field in Iowa. Although struggling to make himself known nationally, he has risen to the top tier in Iowa polls, alongside Sen. John McCain of Arizona and former New York Mayor Rudolph W. Giuliani — thanks largely to his spending.

But challenges for Romney are mounting in Iowa.

The all-but-certain candidacy of former Sen. Fred D. Thompson (R-Tenn.), a star of television's "Law & Order," threatens to undercut Romney's drive for conservative support.

And Romney's domination of Iowa's television airwaves will soon end, when GOP rivals launch their own ad campaigns. Hinting at a brawl ahead, McCain has been mocking Romney, casting him as an unprincipled waffler, most recently on immigration.

Beyond that, Romney's recent two-day swing across Iowa also exposed the difficulty he has responding to questions that require unscripted answers — a challenge he's likely to face again Tuesday in a New Hampshire debate co-sponsored by CNN.

Among the disappointed Iowans was Republican Linda Wessels, 41, of Rock Rapids. At a Romney forum in Sioux Center, her autistic 5-year-old son, Sam, asked the candidate how he would help children with the disorder.

"Cute little guy," Romney responded before launching into a monologue on topics including stem cell research and cloning — but not autism.

"I felt avoidance of the issue," Wessels said.

Retired aerospace worker Gary Steinbeck asked about expansion of the space program, leading Romney into a ramble on science, farming and energy. "He didn't really talk about the space program," Steinbeck said.

And at another forum in West Des Moines, Republican Steven Faux, 54, was left cold after telling Romney that his son's National Guard unit was on the verge of deployment to Iraq. The candidate does not mention the war in his stump speech.

Describing himself as a "worried parent," Faux, a Drake University professor, called the war a "mess" and asked Romney how he would fix it.

Romney responded by voicing support for President Bush's recent troop buildup, saying it had a "reasonable prospect of success." He outlined risks of a quick U.S. withdrawal but offered no hint of how he would proceed if Bush could not stabilize Iraq.

"I thought he gave me a stock answer," Faux told reporters after the forum.

Still, Romney's fast-paced outline of a conservative agenda — fiscal discipline, family values and a robust military — draws frequent, if not fervent, applause. His appearance strikes many as presidential, an image he often tries to enhance by using a giant American flag as his backdrop, as he did last week in Iowa.

With his suntan, swept-back hair and sharply tailored suits, Romney, 60, can also seem "too perfect," as "Tonight Show" host Jay Leno put it — a nicelooking "cardboard cutout" who shuns liquor, tobacco and divorce.

"I can have a good time, but you're not going to hear about it," Romney joked in a recent appearance on Leno's show. "What goes on in Disneyland stays in Disneyland."