Of all the potential late entrants into the Republican nomination contest, the one who gives Mitt Romney the toughest fight is Rick Perry, according to the latest Gallup poll.  If Perry, Sarah Palin, and Rudy Giuliani all get into the race, Perry comes in just two points behind Romney and ahead of Palin and Giuliani, while Michele Bachmann falls to fifth place — well within the margin of error:

Mitt Romney is the leader for the GOP nomination among the current field of official candidates, supported by 27% of Republicans, compared with 18% for Michele Bachmann. However, Rick Perry would essentially tie Romney, with Sarah Palin and Rudy Giuliani close behind, in a scenario in which all three of these undecided candidates entered the race.

Gallup asked respondents to choose among all 11 current and potential candidates, and then asked for their second and third choices. The second and third choices are used to simulate preferences when certain combinations of unannounced candidates are excluded from the race. Three such scenarios include the eight announced candidates plus one of the unannounced candidates. Palin, Perry, and Giuliani finish in no worse than a statistical tie for second place when each is pitted against the eight firm candidates.

If only Perry gets in the race, he starts off five points behind Romney — and five points ahead of Bachmann, who loses four points in the transaction.  Palin comes in at 15 if alone, one point behind Bachmann in a statistical tie for second place, while Giuliani gets 14%, three behind Bachmann for third place, without Palin or Perry.  Romney holds 23% in each model.

The numbers between conservatives and moderates/liberals are also interesting.  Gallup didn’t run separate models for that breakdown as they did with the overall numbers, but if all three jump into the race, Perry ties Romney for the lead with conservatives at 18%.  His support drops off considerably with moderates/liberals, finishing tied for fifth place with Bachmann.  Giuliani wins that demographic at 16%, two points ahead of Romney and Palin.  Somewhat surprisingly, Palin only scores 11% among conservatives, a fourth-place finish behind Romney, Perry, and Bachmann.

Needless to say, the other candidates in the field barely change positions with or without the three late entrants.  All of them had better hope for lightning to strike in Ames in a couple of weeks, at the debate and then at the straw poll.  Without some sort of breakout performance, an entry of any of the three maybes will swamp out any hope of getting the kind of media attention that will build momentum in the fall.