Romney has long enjoyed skirmishing with President Obama as he campaigns for the GOP nomination. But his comments about Gingrich reflect the growing threat he presents to Romney's campaign.
In an interview with Fox News Channel's Bret Baier (see video below), Romney said that he has "by far" the best chance of beating Obama.
"I think to get President Obama out of office, you're going to have to bring something to the race that's different than what he brings," Romney said, after pointing out that Gingrich has spent "30 or 40 years in Washington." "He's a lifelong politician. I think you have to have the credibility of understanding how the economy works. And I do."
Romney was among the most critical of Gingrich after his comments on illegal immigration in last week's debate, saying his stated view that the party needs to consider a more "humane" approach is actually "amnesty."
And Gingrich, despite repeatedly arguing that Republicans should refrain from attacking one another, is returning fire.
In an interview with a South Carolina radio station on Monday, Gingrich called himself a "solid conservative alternative" to Romney.
"I don't claim to be the perfect candidate. I just claim to be a lot more conservative than Mitt Romney," he said.
Gingrich also said he would not "switch my positions for political reasons," suggesting Romney has done just that.
Romney disputed the notion -- also advanced in a new Democratic National Committee attack ad -- that he's flip-flopped on core conservative issues. Romney said the only issue he's changed his mind on is on the government role in abortion.
If he truly was saying anything to get elected, Romney said, he would not continue to defend his healthcare reform law in Massachusetts.
"It's by far the biggest challenge I have in the primary race. And if I were willing to say anything to get elected, wouldn't I just say, 'Oh, it was a mistake.' Because I've watched other people on the stage when they talk about their cap and trade policies, they say, 'Oh, that was a mistake,'" he said. "I'll defend that and I understand it has political implications. And if it keeps me from winning a primary, so be it."