By this point, conventional wisdom is that it's too late for another candidate to enter the GOP presidential contest. In addition to building a campaign organization in crucial primary states, a new entrant would have to develop a network of fundraisers, corral endorsements, and find a place within the primary electorate itself. And given the extent to which most party actors have already committed themselves to one candidate or another, it's hard to imagine success for a latecomer to the race.
Arguing against this view is Crystal Ball columnist Rhodes Cook, who isn't so sure that the situation is hopeless for someone who wanted to enter the Republican primary at this stage. As he sees it, the "elongated" shape of the GOP primary calendar creates the space for a late entrant to the race:
But the elongated layout of the nominating calendar this time provides the opportunity for a late-starting candidate to emerge. Should Mitt Romney stumble badly in the January events in Iowa, New Hampshire, South Carolina and Florida, another establishment Republican could enter the race in early February and still compete directly in states with at least 1,200 of the 2,282 or so GOP delegates. Many of them will be up for grabs after April 1 when statewide winner-take-all is possible. […]
To be successful, a late-starting campaign needs to feature a candidate with considerable fund-raising and organizational ability who is capable of quickly grabbing national attention. Charisma helps, as does a campaign message that can evoke widespread support. Robert Kennedy fit the bill in 1968, and there are arguably a few prominent Republicans on the sidelines this time who could mount a competitive, late-starting candidacy in 2012.
Again, even if you could find a candidate with the charisma and popularity to energize Republican voters, they would still need the resources and personnel necessary for a credible campaign, and that's a tall order.
What's more, if I were a Republican, I'm not sure that I would want a late entrant, even if he or she met my criteria for an acceptable nominee. Insofar that there's any benefit to a protracted primary campaign, it's that it gives you a chance to see if a candidate can survive in real-world condition. A candidate might look good on paper, but fall flat when it comes time to compete in an actual election (see: Tim Pawlenty and Rick Perry). Moreover, extended primaries give everyone a chance to vet candidates and ensure that there isn't anything that could damage a candidate in the general election. If a late entrant were to win the primary, there's no guarantee that they would be properly vetted, which could lead to disaster.
In any case, this isn't something to worry about because the odds of it happening are very low; the primary roster is set, and when the GOP chooses its nominee next year, it will be a familiar face.