Now that he’s out, the GOP race hinges on whether former Arkansas Gov. Mike Huckabee decides to get in. His former campaign chairman, Ed Rollins, says that he will. If he does, he seems a prohibitive favorite to win the Iowa caucuses, a likely loser in the New Hampshire primary (to Mitt Romney), thus setting up a showdown between the Christian and the Mormon in South Carolina. Huckabee wins that in a walk and probably sweeps the remaining Southern primaries and becomes, de facto, the GOP nominee. Republican presidential candidates who carry the South in the primaries win the GOP nomination.

Assuming Huckabee does run (and we assume that), the departure of Haley Barbour is especially bad news for Mitt Romney, who needed another “southern” candidate in the South Carolina primary to draw votes away from Huckabee. Head-to-head against Huckabee, Romney has virtually no chance of winning anything but the coastal counties in South Carolina. With Barbour in the mix, there was at least the possibility that some of Huckabee’s good ole boy voters would wander over to the Barbour column.

The big question now is: what happens if Huckabee doesn’t run?