It looks like it has to be Gingrich and Romney. The others have either faded from view or are suffering serious foot wounds. Perhaps the winner of the primary battles will choose the loser for Veep. It's clear what their campaign symbol will be no matter which one runs, so the only question left to be settled would be which one gets to be shown on the right side of the posters.
Newt got his bid for the right side in this recent exchange:
Reporter: What can we tell these young people who are occupying Wall Street?
Gingrich: Go get a job right after you take a bath.
Reporter: There are 14 million people looking for work. What if they can't find a job?
Gingrich: Let them eat cake.