John Sides explains:
In The Party Decides, the political scientists Martin Cohen, David Karol, Hans Noel, and John Zaller argue that one thing is likely to make political parties nominate a centrist: losing. Specifically, the longer a party is out of power—that is, the more presidential elections it has lost in a row—the more likely it will nominate a moderate candidate. Parties that have been out of the White House for only a short time are more willing to nominate a candidate closer to the ideological pole.
A shorter version of my rule of thumb: it gets worse before it gets better.
The Dish 