Once again, the Republican primary race has a new national frontrunner, and once again, his name is not Mitt Romney … or perhaps Not Romney, depending on your point of view. Rasmussen’s new national poll puts Rick Santorum ahead of Romney by twelve points in a four-man race — and by 21 points in a two-man race:

Follow the bouncing ball. Former U.S. Senator Rick Santorum has now bounced to a 12-point lead over Mitt Romney in the race for the Republican presidential nomination. The latest Rasmussen Reports national telephone survey of Likely Republican Primary Voters finds Santorum with 39% support to the former Massachusetts governor’s 27%. Former House Speaker Newt Gingrich follows from a distance with 15% of the vote, and Texas Congressman Ron Paul runs last with 10%. Three percent (3%) like some other candidate in the race, and six percent (6%) are undecided. Just over a week ago, it was Romney leading the pack with 34% after his win in the Florida Primary, followed by Gingrich 27%, Santorum 18% and Paul 11%. But then last week Santorum swept GOP caucuses in Colorado and Minnesota and a non-binding primary in Missouri as Gingrich continues to stumble in the race to be the conservative alternative to Romney. Perhaps more tellingly, Santorum now trounces Romney 55% to 34% in a one-on-one matchup among likely GOP primary voters. This is the first time any challenger has led Romney nationally in a head-to-head match-up. Santorum also leads Romney head-to-head in Michigan. Nationally, Romney still wins 47% to 38% paired against Gingrich and 47% to 42% running against only Paul.

Some have suggested that Santorum is suffering from a gender gap with Romney. That could be true in polling overall in comparing head-to-head matchups against Barack Obama (and Democrats usually win among women anyway), but it’s not true in Rasmussen’s crosstabs. Santorum beats Romney by eleven among women, almost exactly the same margin as his overall lead. He beats Romney by twelve among men. Santorum also does surprisingly well among younger voters (18-39), almost doubling up on Romney 37/19 with Ron Paul second at 20%. Romney does better with seniors, but still falls six points behind Santorum, who leads 39/33.

Ideologically, Santorum gets a majority of “very conservative” voters (53%), with Romney and Newt Gingrich tying at 17%. Santorum barely edges Romney with “somewhat conservative” voters at 37/34, but Romney easily wins “others,” 30% to a tie between Santorum and Paul at 22%. But most fascinating of all, Santorum leads Romney among both Republicans (38/28) and independents — by 17 points, 42/25. He wins more narrowly among those certain of their vote at 38/32, which is a point to watch in the next two weeks as Arizona and Michigan hold their primaries.

In the head-to-head battle between Romney and Santorum, the disparity grows even more. Santorum wins Republicans by 19 (54/35) and independents by 26 (58/32); he wins very conservative voters by 48 points, 70/22, and somewhat conservative by a majority, 51/37. Santorum also wins the favorability battle over Romney, but not by a lot, having a +51 (75/24) to Romney’s +36 (66/30). Santorum does better among independents on this measure, too, with a 72/26 to Romney’s 64/35.

Santorum has indeed arrived. The real test will be whether he can hold up to the scrutiny.

Update: Radley Balko tweeted this link with the description, “Things I didn’t expect to read today.” Megadeth frontman Dave Mustaine has endorsed Santorum:

Looking at the Republican candidates, I’ve got to tell you, I was floored the other day to see that Mitt Romney’s five boys have a $100 million trust fund. Where does a guy make that much money? So there’s some questions there. And watching Newt Gingrich, I was pretty excited for a while, but now he’s just gone back to being that person that everybody said he was – that angry little man. I still like him, but I don’t think I’d vote for him. Ron Paul… you know, I heard somebody say he was like insecticide – 98 percent of it’s inert gases, but it’s the two percent that’s left that will kill you. What that means is that he’ll make total sense for a while, and then he’ll say something so way out that it negates everything else. I like the guy because he knows how to excite the youth of America and fill them in on some things. But when he says that we’re like the Taliban… I’m sorry, Congressman Paul, but I’m nothing like the Taliban. Earlier in the election, I was completely oblivious as to who Rick Santorum was, but when the dude went home to be with his daughter when she was sick, that was very commendable. Also, just watching how he hasn’t gotten into doing these horrible, horrible attack ads like Mitt Romney’s done against Newt Gingrich, and then the volume at which Newt has gone back at Romney… You know, I think Santorum has some presidential qualities, and I’m hoping that if it does come down to it, we’ll see a Republican in the White House… and that it’s Rick Santorum.

That’s a little counterintuitive, isn’t it? Also, Matt Lewis has one word of advice for Santorum as the scrutiny increases: abstinence.