There hasn’t been a sitting President since before FDR who won reelection after experiencing a serious primary challenge.
The threat of such, and laying it out now, is IMHO a valid and workable check on Mittens returning to his old ways should Conservatives hold their noses and support him in an ABO scenario. So long as the one(s) making the threat are trusted Conservatives and there are other clear indicators (like who he picks as his veep) that he plans to go against his past track record and govern from the right.
If that nominee realizes in time that he must either win or enter the Ford/Bush41/Dole/McCain/ political perdition of “nobody gives a damn about you but you you loser” he will do all he can to kiss Tea Party Butt and promise, in writing, to do as they wish and to then step down in 2016 to allow a real conservative to take it from there..
If not, the GOP is finished and then it will be either Civil War 2 or a successful third party.