To answer this question, a few
national pollsters have tested Romney against a sole conservative alternative.
They do this by first testing all the viable Republican candidates (Bachmann, Cain, Gingrich, Huntsman, Paul, Perry, Romney, and Santorum), and then following up this full-slate question by polling Romney vs. one other candidate (Cain, Gingrich, or Romney).
If non-Romney voters are as a group solely looking for a Romney alternative, we would expect Romney's support relative to a given conservative alternative to drop dramatically from the full matchup to the followup one-on-one question.
Mathematically we can test this through the following equation:
Romney split field advantage = Romney support / (Romney support + conservative alternative support) * 100 in the one-on-one test - Romney support / (Romney support + conservative alternative support) * 100 in the full trial heat.
This equation gives us Romney's percentage of the vote vs. the conservative out of 100% with undecideds allocated.
A negative value means that voters who do not vote for Romney or the conservative alternative in the full candidate lineup are mostly part of the "anyone but Romney" crowd. A positive means that Romney would not necessarily be hurt once the field comes down to one other candidate and him.
As the table below shows, Romney support relative to the conservative alternative has remained about the same in the full and one-on-one matchups.
In the 8 times the Romney one-on-one matchup has been polled, Romney has done on median only 2% worse in the one-on-one than in the full trial heat. The biggest difference was an early November NBC poll that had Romney's support dropping from 74% to 63% in a one-on-one against the now weak Rick Perry.
While this number may be close to being statistically significant, it is not substantially significant. If non-Romney voters in the full trial heat were absolutely against him, we would expect him to do far worse than 2% in the one-on-one.
Does this mean that Romney is going to win in a one-on-one against one conservative? No.
For one thing, we are talking about support relative to the full trial heat. When Romney trails the conservative in the full matchup, he trails them in the one-on-one. In the most recent Quinnipiac poll matchup, he held 46% of the vote relative to Gingrich in the question including all the candidates and 44% in the one-on-one.
It is only when Romney leads in a poll of all the candidates should we expect him to lead in a one-on-one poll. An example of this phenomenon is an early November NBC poll when Romney held 51% of the vote in both the full trial heat and one-on-one against Herman Cain.
I should also point out that any one candidate (e.g. Herman Cain) might have his/her support go overwhelmingly to one candidate. A Selzer and Co. Bloomberg Iowa poll
shows Herman Cain's voters going to Newt Gingrich and Rick Perry, not Mitt Romney*.
Still, the evidence from the national polls suggests that Romney is not greatly benefitting greatly from a split field. If he is leading when there are eight candidates in the race, he will still be leading when there are only two.