It's been too late for some time now

Colorado political consultant Eric Sondermann, in Sunday's Denver Post:

At the end of 2011, I said that I still thought there was one shot in three — no more than that — that the nominee would be somebody not currently in the race. Every time the Romney freight train gets slowed down or temporarily derailed, that brings up that option again of whether Romney is ultimately going to be able to close this deal. I still say that it is only one shot in three. But one shot in three is a lot higher than in any normal political year.

No, there will not be another candidate. The field is set and has been for some time now. Jonathan Bernstein and Josh Putnam dismissed the idea of a late-entering candidate three months ago, and they were repeating themselves then. There are three main reasons:

1. Filing deadlines: It is now too late to enter roughly half the state primaries and caucuses. Yeah, that makes winning a majority of delegates kind of challenging.
2. Romney's not that unpopular: Sure, the contest has been a bit longer and bloodier than Romney wanted, but polling consistently shows that a broad majority of party voters would be okay with him (if not unabashedly enthusiastic about him) as the nominee.
3. The Republican Party does not have a death wish: The last time a party nominated a presidential candidate who had not participated in primaries and caucuses was 1968, and it led to a massive rift within the Democratic Party and a number of profound reforms to the nominating system. If the Republican Party is at all concerned about maintaining the loyalty of its most active voters, it is not going to diss them by nominating someone for whom nobody campaigned.