Here’s what you need to know about the barrage of Iowa polls from the last several days. Mitt Romney, Ron Paul, and Rick Santorum are running neck-and-neck for the lead in the Hawkeye State. But there’s still a lot of uncertainty, and one of those three could still slip into fourth place or lower — meaning there’s still a good chance for a surprise that could complicate Romney’s march to the nomination.

Taking those one at a time: Public Policy Polling reported an Iowa survey late last night, joining the Des Moines Register’s gold-standard Iowa Poll, which released their final sounding on Saturday. Nate Silver’s projection model tosses those two into the pot with all previous polls, and winds up with close to a dead heat: his model now shows Romney at 21.0%, Paul just behind his at 20.6%, and Santorum closing fast at 19.3%. After that, Newt Gingrich is still in fourth place, followed by Rick Perry and then Michele Bachmann.

Perhaps the most important point of all: It probably doesn’t matter what order the top three finish in, as long as Romney, Paul, and Santorum occupy the top three spots. The biggest consequence will be that the other three campaigns will be almost certainly irreparably damaged. If Perry does finish fifth, or even a weak fourth, I expect he may likely drop out. Normal candidate incentives would also push Bachmann and Newt to drop out at this point, but it’s not clear whether normal incentives apply, particularly for candidates who are running only to enhance their brand for the conservative marketplace.

As for the top three, what matters isn’t the results in Iowa, but how those results are spun and how they affect the coming states. Three things produce post-Iowa spin: the raw difference between expectations, generally determined by polling, and actual results; how party actors react; and media biases. The first, the expectations game, will favor Santorum, since just a week ago he was still in a jumble for fourth place. Media biases that matter favor new things, unexpected things, and keeping the contest alive for as long as possible; all of those will tend to help Santorum. Another dynamic to watch: Will Santorum’s surge persuade undecided conservatives to rally around him? If so, a strong pro-Santorum spin would lead to a big bounce in New Hampshire and make him competitive in South Carolina and perhaps beyond.

Last point: Polling the caucuses is extremely difficult, far harder than polling a general election and even more difficult than polling a regular primary. No one really knows who will show up tomorrow night, and so all polling contains quite a bit of turnout-related guesswork. Moreover, the polling all points to a lot of uncertainty, and so last-minute decisions affected by all sorts of things could upset even the best of pre-caucus polls. For more on the uncertainty, see Pollster’s Mark Blumenthal here, Nate Silver here, and the Iowa Poll’s J. Ann Selzer here.

Bottom line: it sure looks as if we have a Romney/Santorum contest (with Paul tagging along) coming out of Iowa, with Romney having a huge head start and remaining the best bet to win the nomination. But this is all in flux, and in particular I’m still watching to see whether Rick Perry can manage to salvage a strong fourth and remain alive to fight in South Carolina.