Just two days out from the Iowa caucuses, it's Rick Santorum's turn.
"Our backgrounds are quite different," Romney told reporters crowded into a back room at the Family Table Restaurant here. "Like Speaker Gingrich, Senator Santorum has spent his career in the government in Washington. Nothing wrong with that. But it's a very different background than I have."
Romney's mild criticism came in response to a question about how he would persuade voters to back him over Santorum as polls show the former Pennsylvania senator surging.
A lot can change in just a few days here, where the race to win the approval of Iowa Republicans is as unsettled as ever. Late last week, those questions were about Ron Paul. Earlier that week, Newt Gingrich. A few months ago, Rick Perry.
Romney has continued with the same optimistic campaign speech. As large crowds have packed into his events, Romney has praised America and attacked President Barack Obama while largely ignoring his GOP rivals.
"I don't think I've spent a lot of time trying to describe differences on policy and detail on myself and the other candidates, but instead I focus on the things I believe," Romney said Sunday.
Not quite. Romney has been much sharper with the rivals when his campaign worried they could have the money, organization and potential support to challenge him for the Republican presidential nomination. Just last week, Romney compared Gingrich's failure to earn a spot on Virginia's primary ballot to a 1950s sitcom.
"It's more like Lucille Ball at the chocolate factory," he said when a reporter asked him about Gingrich's troubles.
Romney's campaign has also attacked Gingrich in direct mail pieces, and his allies are airing anti-Gingrich ads in Iowa and early primary states, including South Carolina and Florida.
In the face of the attacks, Gingrich has fallen back in early-state and national polls. Speaking to reporters in Iowa Sunday, Gingrich complained that his campaign had been "Romney-boated," referring to the third-party group Swift Boat Veterans for Truth that helped sink Democrat John Kerry's presidential candidacy in 2004.
For now, Gingrich and Perry are fighting with Paul and Santorum, while Romney has continued his campaign as planned. He's made a handful of campaign stops every day since last Tuesday, with a break on New Year's Eve to spend some time campaigning in New Hampshire. He plans to spend caucus night in Iowa before returning Wednesday morning to New Hampshire, which holds the nation's first primary.
At his stop in Atlantic, scores of reporters, photographers and camera crews packed into a small restaurant, making it impossible to tell how many Iowa voters were seated at the tables and waiting to see Romney.
"You can't sit there," one of the managers said testily to a reporter perched on the counter next to the cash register. "People are trying to pay."
Romney's staff ended up bringing him into the event through the kitchen because he wasn't able to get through the front doors.
The voters who did make it inside weren't all pleased. "It's awful in here," said Cathy Dowhey, 51, who plans to speak on behalf of Romney at her caucus in Atlantic. She braved the crush to get Romney's signature on the package of information the campaign mailed to her so she could prepare for the role.
Some didn't make into the restaurant. Don McLean, a local pastor who is leaning toward backing Santorum, looked sideways at his 14-year-old daughter as he stood outside in freezing temperatures.
"It's too crowded," McLean said as he stood a few feet from the front door.
He waited a few more minutes. With still no sign of Romney's arrival, he got into his car and drove away.
"We're going home to watch some football," he said.