“ruling Republicans” – someone said above – actually, the Democrats control the Federal government
Romney is clearly dull and boring, though arguably that might be an attraction. Many people are tired of all the excitement of the last ten years and would prefer a period of dull and boring, peace and prosperity.
Santorum is focused on social issues, which not only have nothing to do with the office of the President (e.g. abortion, which is entirely a matter for the Congress and the courts), and his only economic credential was being a well know earmarker.
Gingrich’s personal life is weird (he’s on his third marriage, the first two failed because of his misbehavior), and he keeps changing his views. Romney has been fairly consistent, especially on issues pertaining to the presidency. His shifts on abortion and gays were fairly slight – note that he’s hardly being embraced by left wing types.
Economically, Romney is closer to the “tea party” than Santorum or Gingrich.
He might be clever enough to argue that America wants a boring president who gets the job done, and Romney could probably embrace most of the other Republicans platforms and saying that he’s the one to get them enacted.