As expected, the debate over Social Security played a fairly important role in the debate for Republican presidential candidates last night. Indeed, almost immediately after the event began, the field’s top two candidates, Rick Perry and Mitt Romney, went toe to toe over the issue.

For the Romney camp, this probably seemed like good news. The more the light shines on Perry’s outside-the-mainstream views, the easier it is for the former Massachusetts governor to argue that the Texas governor is unelectable.

It’s why Perry has moderated his tone a bit, assuring today’s seniors last night that the “program is going to be there in place” for them. But Romney, not surprisingly, sees value in exploiting a perceived weakness.

“[T]he term ‘Ponzi scheme’ I think is over the top and unnecessary and frightful to many people. But the real issue is in writing his book, Governor Perry pointed out that in his view that Social Security is unconstitutional, that this is not something the federal government ought to be involved in, that instead it should be given back to the states.

“And I think that view, and the view that somehow Social Security has been forced on us over the past 70 years that by any measure, again quoting his book, by any measure Social Security has been ‘a failure,’ this is after 70 years of tens of millions of people relying on Social Security, that’s a very different matter.”

The back and forth continued a bit, with Romney pressing Perry on whether he thinks the program is unconstitutional, and Perry responding, “If what you’re trying to say is that back in the ’30s and the ’40s that the federal government made all the right decision, I disagree with you. And it’s time for us to get back to the Constitution.”

Romney added that Perry is “scaring seniors,” and rhetoric that suggests “Social Security should no longer be a federal program and returned to the states and unconstitutional is likewise frightening.”

Obviously, for much of the mainstream, Romney was on the smarter side of this dispute, and his exchange made Perry look like something of an extremist.

But the point I keep coming back to is the strength of extremists in Republican politics, and how far the entire party is from the American mainstream. Consider this item from yesterday, run before the debate:

During his Monday show, Limbaugh warned the 2012 Republican field not to use Perry’s remarks against him. He specifically named Minnesota Rep. Michele Bachmann and former Massachusetts Gov. Mitt Romney.

“I’ve not endorsed anybody and this is not an endorsement” Limbaugh said. “But be very careful if you start attacking Rick Perry on Social Security and the ‘Ponzi scheme.’ There are too many of you out there who have already said that yourselves — Mitt Romney. Mitt, you have already called it a Ponzi scheme. And worse.”

“I’ve got a whole list of people here — media and outside — in politics who have referred to Social Security as a Ponzi scheme,” Limbaugh went on. “And … I would like to warn everybody: Be careful here because you’re pandering to the media.”

As a factual matter, as best as I can tell, Romney hasn’t called Social Security a “Ponzi scheme.” But the larger point is, Romney’s argument is likely to resonate with sane people everywhere, while Perry’s argument is likely to resonate with the kind of folks who participate in Republican primaries and caucuses.

Romney is left telling GOP voters how much he loves a program Republicans have labeled “socialism” since its inception. He wants to make an electability argument, but he’s doing so by voicing his support for the pearl of the New Deal. We’ll have to see how that turns out for Romney, but assuming rank-and-file GOP voters will repel is likely a mistake.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.