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And again, in the polls, Rick Santorum is ahead of Romney in Michigan. Santorum is a social issue conservative with a blue-collar background - and proving to be a tougher opponent to Romney than ever expected.
Here's NPR's national political correspondent Mara Liasson, with a look at how much of a challenge the former Pennsylvania senator presents to the former Massachusetts governor.
MARA LIASSON, BYLINE: What's the best way for Mitt Romney to stop Rick Santorum? For the answer, we went to someone who's done it before. In 2006 in Pennsylvania, Democratic strategist Saul Shorr helped Bob Casey defeat then-Senator Santorum in a landslide. Santorum lost by 18 points.
But Shorr says that was a general election. In a Republican primary, Mitt Romney will have a much harder job.
SAUL SHORR: Here's Romney's challenge - you know, in a lot of these primaries you're dealing with an electorate that's made for Santorum. They have questions about Romney's conservatism, and they don't have any questions about Santorum's. Look, when Romney's folks took out Newt Gingrich, you know, it was really like shooting fish in the barrel.
LIASSON: Unlike Gingrich, Shorr says, Santorum is not a target-rich environment. But Charlie Black, an outside adviser to the Romney campaign, isn't so sure. He says people are overestimating Santorum's surge.
CHARLIE BLACK: The polls you see right now reflected the bounce, the halo effect he got from winning three nonbinding events last week. Santorum really hasn't had a glove laid on him yet, but his record and his position on the issues will be examined now.
LIASSON: And the Romney team is just getting started, with millions of dollars in ads in many of the upcoming primary states, including Romney's native state of Michigan, where a loss to Santorum would be humiliating. The ads define Santorum as a friend of K Street, and a big spender.
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UNIDENTIFIED ANNOUNCER #1: Rick Santorum's been in Washington so long, he's called the ultimate Washington insider. Santorum voted to raise the debt limit five times, and for billions in wasteful earmarks.
LIASSON: The Santorum campaign is trying to inoculate itself against the assault.
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UNIDENTIFIED ANNOUNCER #2: Mitt Romney's negative attack machine is back, on full throttle. This time, Romney's firing his mud at Rick Santorum. Romney and his superPAC have spent a staggering $20 million, mostly attacking fellow Republicans.
LIASSON: Hogan Gidley is Santorum's national communications director.
HOGAN GIDLEY: We're not going to sit back and take incoming from the Mitt Romney campaign that's dishonest and untruthful and negative. That's not what the country wants to hear. I think they've roundly rejected that to this point - as you can tell by Mitt Romney's troubling favorable ratings, while Rick Santorum sits in the 70 percentile.
LIASSON: In Michigan, for instance, Santorum's favorable ratings are much higher: 67 percent to Romney's 49. And that's why, says Dr. Richard Land - the head of the Southern Baptist Convention Ethics and Religious Liberty Commission - Romney needs to be careful.
DR. RICHARD LAND: If he starts trying to attack Rick personally the way he attacked Newt, it's going to be like, you know, big, mean Mitt jumping all over the choir boy. There could be a real backlash.
LIASSON: And there's something else fueling Santorum's rise: Social issues have returned to the debate, thanks to President Obama's decision to require contraceptive coverage at religiously affiliated institutions. Land says that gives Santorum an edge.
LAND: Because people know about Romneycare, and they know that it was similar to Obamacare. And when the president takes off his mask and we see the ugly business-end of government coercion - as we did last week - it reminds social conservatives of what's at stake.
LIASSON: In a Republican primary, Romney can't run to Santorum's left by arguing that Santorum is too conservative to beat President Obama - even though that may be true. And unlike Gingrich or even Rick Perry, Santorum doesn't offer Romney much room on his right. Saul Shorr thinks the Romney campaign is in a tough spot.
SHORR: I'm sure that they're poring over their polling, really trying to look for where to go against Santorum. Yes, he can try to paint Santorum as an insider in Washington, but he's really an insider of the corporate suite. I just think that's a tough sell. And in some way, shape or form, it may be that Romney's positive ads are going to matter as much as anything else.
LIASSON: Now, that's an odd notion - that this year, positive ads could make the difference. But plenty of Republicans say Romney has to do more to lay out a positive vision for the future. One way or another, Charlie Black is still counting on Romney's superior resources.
BLACK: You're going into a period where the race becomes more national. There are actually 13 events - including in some large states - in the next three weeks. One reason that Romney has withstood the challenge of four or five or six candidates already is, he has a national political organization and a national fundraising organization. Nobody else does.
LIASSON: Black predicts Romney's organization and money will prevail in the upcoming states. It's certainly true that when Romney has brought this advantage to bear, he's won - see Florida. When he hasn't spent big, he's lost - see Colorado, Missouri and Minnesota. But it might take more than financial advantage for Romney to regain his front-runner status.
Mara Liasson, NPR News, Washington.
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