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.