It seems there is room for another candidate--say, Mitch Daniels, for example, or even Bobby Jindal--to make a late entry into the Republican primary field. Consider Rasmussen's latest survey:
While many pundits have taken to describing the race for the Republican nomination as a two-man competition, a third of all voters nationwide think it would be good for the GOP if someone else jumped into the fray.
The latest Rasmussen Reports national telephone survey finds that 34% of Likely U.S. Voters think it would be good for Republicans if another candidate entered the race for the party’s presidential nomination. But nearly as many (31%) say it would be bad for the party, while 24% believe it would have no impact. Twelve percent (12%) are not sure.
More importantly: "33% of GOP Voters Say It Would Be Good If New Candidate Entered Presidential Race."
A large chunk of voters are still looking for another candidate. Back in June, NBC reported: "[W]ith about eight months until the first GOP nominating contests, less than half of Republican primary voters — 45 percent — say they are satisfied with their current crop of presidential candidates, according to a new NBC News/Wall Street Journal poll."
The fact that there is little difference between those June numbers and today's Rasmussen numbers does not speak well for the current candidates in the field, who have been campaigning daily in the months since then.