Much anti-Romney analysis claims that the multiple candidates running against Romney are splitting the Republican vote and allowing the "Massachusetts Moderate" (not sure if this phrase has been trademarked by Gingrich yet or not) to beat a divided field. Polling before might have backed up this claim (when various not-Romneys beat Romney in a one-on-one polling match-up), but it now tells a different story.
For example, recent YouGov polling showed Romney beating Santorum 58%-42% and beating Gingrich 66%-34% nationally. A CNN poll of Republicans nationwide released on Friday showed Romney beating Santorum 60%-37% and beating Gingrich 59%-37%. This trend also carries over into many statewide races. In South Carolina, a PPP poll shows Romney with a five-point lead against the whole field. Match-ups where Romney is pitted against a single other Republican see his margin of victory grow considerably. He leads Gingrich 48%-37%, Paul 63%-28%, Perry 56%-31%, and Santorum 48%-39%.
Many supporters of one not-Romney feel more comfortable with Romney rather than another not-Romney. So, at this point, it seems as though a full field could be hurting Romney more than helping him. A multiplicity of opponents can attack Romney (sometimes in contradictory ways) and hope to pull his polling numbers down enough to allow one of the not-Romneys to win. At this time, however, polling seems to suggest that Republicans would rally around Romney more than they would around any of his GOP rivals.