Former Sen. Rick Santorum
Santorum is the big question mark among the ex-candidates for the GOP nod. Santorum was one of Romney’s harshest critics in the primary, and his repeated attacks on Romney’s conservatism would make it difficult for him to pull off an about face and endorse Romney.
Although Romney has signaled he is interested in meeting with Santorum to discuss an endorsement, so far there has been no such rendezvous. Santorum’s name has also been floated as a potential running mate for Romney, but the likelihood of that prospect appears to be small.
Still, Santorum is in solid shape to be a front-running candidate in 2016 if Romney loses in November, and he has been careful to make clear to conservatives that he’s a team player with the party’s best interests at heart.
"I pledge to you, I will be all-in between now and November to make sure we elect Republicans, conservatives up and down the ticket," Santorum said.