Romney’s biggest obstacle could come from Sarah Palin, whose decision about whether she will run at all remains a mystery. She isn’t polling all that well in our primary matchups anymore, with more Republicans coalescing around the announced candidates. But Palin would split general-electorate Republicans, with Romney at 46% to her 41%. As she usually does when the two are pitted head-to-head, she would give Obama his biggest lead among independents, 51-26, with Palin at 15%. Overall, Obama would run away with the election, 47-26-21 over Romney and Palin.

The main problem for Romney is that Obama holds his base, both Democrats and the independents who are inclined to vote for him anyway, better than Romney does his. Most of the peeling-off is among Republicans (at least against Palin, Trump, and Paul) and independents who might ordinarily lean Romney’s way if he were the only other choice than Obama. These independent candidates earn only 4-9% of the Democratic vote, and Obama gets 41-51% of the independent vote, not much less than the 48-59% he gets against just the five Republicans we tested against him head-to-head. By contrast, while Romney is the strongest head-to-head with independents, trailing the president by nine points, he gets a measly 26-35% of the independent vote in these three-way match-ups. The only third-partier who would help Romney in that department is Bloomberg, and Romney would still lag by six with independents, and by 10 to 25 points versus the others.