With the Iowa caucuses convening tonight, a few points to consider:

Who shows up? Jim discussed the Des Moines Register poll Saturday night, which has a very good track record and showed Mitt Romney leading with Ron Paul in second — but Rick Santorum in second in the last two days of polling. DMR pollster Ann Selzer walks through how the results change if the sample is weighted differently:

EVANGELICALS – In 2008, they were 60 percent of those who participated in the entrance poll at the Republican caucus and famously handed Mike Huckabee the win. This year, our polls are showing far fewer likely Republican caucusgoers identifying themselves as born-again or evangelical Christians – about one in three.

When we weight our data from the last two days in the field to match 2008, Rick Santorum wins with 25 percent, Mitt Romney is second at 20 percent, and Ron Paul has 16 percent. Michele Bachmann, Newt Gingrich and Rick Perry barely break into double digits. Each is within a point of one another.

SENIORS – In 2008, they made up 27 percent of the entrance poll respondent pool. We are showing them under 20 percent in our polls this year. When we weight our data from the final two days of polling to match 2008, this greatly benefits Romney, who would rise to 26 percent and a 7 point lead over Santorum, 19 percent. Paul drops to third place, with 17 percent.

Selzer goes on to explain that a surge of independents helps Paul (though it doesn’t put him quite over the top according to her data); PPP’s photo-finish poll, showing Paul with a statistically trivial lead, is based on 24% of caucusgoers being independents and Democrats; this seems unlikely, but not impossible.

The fight for fourth. If Romney, Paul, or Santorum don’t finish in the top three, it would obviously be a huge loss in the expectations game (probably fatal, in Santorum’s case). There’s an old saw about there being “three tickets out of Iowa,” but this year there’s probably a fourth ticket; if Newt Gingrich, Rick Perry, or Michele Bachmann comes in fourth, he or she will declare it a victory and continue the campaign. Nate Silver’s forecasting model — which averages polling data with weights designed to factor in pollsters’ track records — shows Gingrich holding steady in fourth place after factoring in the two polls (Insider Advantage and ARG) that came in after PPP’s. It’s possible, though, that Perry can sneak past Newt with a strong ground organization.

The weather. As Stacy noted on Christmas Day, it’s been posited (by Mike Huckabee, among others) that a snowstorm would help Ron Paul, the idea being that his younger and more dedicated core of supporters would show up even when others decided to take a pass on the caucuses. Today’s weather in Des Moines is expected to be cold, but clear.