Romney and Perry debating
Mitt Romney, left, and Rick Perry argued a point during a recent Republican presidential candidates debate in Tampa, Fla. Reuters

Mitt Romney is catching up to Rick Perry in at least one poll: the Gallup Positive Intensity Score.

Gallup calculates a candidate's Positive Intensity Score by taking the percentage of respondents with a strongly favorable opinion of the candidate and subtracting the percentage of respondents with a strongly unfavorable opinion. The resulting number indicates the strength of a candidate's appeal to voters, as well as how polarizing they are.

Perry continues to tops the list with a score of 24 -- one point down from his score of 25 when Gallup last calculated on Aug. 28 -- but Romney's score improved substantially in that same period, from 11 to 16. Romney is now fourth in the poll, behind Perry, Herman Cain and Rudolph Giuliani, compared to his sixth-place finish last month. Among the frontrunners, he is second, as Giuliani hasn't actually entered the race and Cain has single-digit support in regular opinion polls.

This is especially noteworthy because Romney was the only candidate among the 10 included in the poll whose score increased by more than one point. Michele Bachmann, Herman Cain, Jon Huntsman, Sarah Palin and Rick Santorum all slipped, with Palin losing six points, Cain losing five, Bachmann losing three, and Huntsman and Santorum losing two apiece. Giuliani and Ron Paul gained one point each. Newt Gingrich was the only candidate whose score (7) did not change.

Huntsman now has the unwelcome distinction of being the only candidate with a negative Positive Intensity Score, meaning there are more people who strongly dislike him than people who strongly support him.

The Positive Intensity Score is not the most useful measure of a candidate's chances, because it only indicates the intensity of their support, not the amount of it. Cain, for example, has an excellent score, but given his single-digit poll results, his chances of winning the nomination are close to nil.

However, while it is not a reliable indicator of who has the most support, the Positive Intensity Score does provide a decent picture of overall trends in the race.

It shows that Romney is gaining ground, that Perry is staying more or less steady, and that the other candidates -- including Bachmann, who was once considered a frontrunner -- are fading. It is further evidence that the Republican primaries have become a two-man race, and also that, while Perry maintains a sizable lead for now, he can't rely on a slow-and-steady-wins-the-race strategy. If he wants to stay ahead of Romney, he will have to attack him head-on.