Rick Santorum and Michele Bachmann barely seem worth mentioning at this point.
Which leaves... Romney. It may be one reason that more and more key GOP figures are coalescing around him.
Romney has always been more the choice of the Republican establishment, but now he's starting to win endorsements from key conservatives outside the establishment as well.
South Carolina Gov. Nikki Haley (one of Sarah Palin's "Mama Grizzlies") is one of the most high-profile recent examples. And he has the endorsement of Christine O'Donnell – the tea party favorite who won the Delaware primary (and lost the general election) despite having the GOP establishment against her.