First, the split in the Republican Party is no longer between conservatives and moderates, but between members of the party who are very conservative and those who are only somewhat conservative. The days of Rockefeller Republicans are long gone. Close to half of Iowa caucus-goers thought of themselves as very conservative; a third said they were somewhat conservative. Fewer than a fifth were moderates, including a very tiny (and brave) group of self-described liberals.

Romney’s constituency is Republican Classic. He was the candidate of the “somewhat conservatives” and did well with the moderates, particularly moderate Republicans. (Moderate independents went strongly for Ron Paul — and thanks to Mike Dimock of the Pew Research center for sharing his insightful analysis for NPR of the difference between moderate independents and moderate Republicans.) Romney trailed badly with very conservative voters, running well behind Santorum in that group. Romneyites are much older: He was strongest among caucus-goers over 65 — which is presumably hopeful news for him in the Florida primary at the end of the month — and he also did well among voters between 45 and 64. But he did very poorly among voters under 45.

Rick Santorum, as he hoped to, won a lot of the same vote that Mike Huckabee carried four years ago. Santorum is clearly the right-to-life candidate: He carried voters who listed abortion as their deciding issue by a landslide. He was definitely the surge candidate: He handily won voters who said they decided in the last few days, though Romney did relatively well in this group, too, suggesting he helped himself with his final campaign push. Interesting, Santorum ran quite evenly across age groups; his constituency was slightly younger than caucus-goers as a whole. Santorum ran well ahead of Mitt Romney among white evangelical voters, but he had to share them with Rick Perry, Ron Paul, Newt Gingrich and, to a lesser degree, with Michele Bachmann. Bachmann and Gingrich’s fades were key to Santorum’s breakthrough.

If Santorum should fall short of victory — and at this moment, he is still very much in the hunt for first place — it will be the fracturing of the evangelical and very conservative vote that held him back. Consider Calhoun County in northwestern Iowa. That county voted overwhelmingly for Huckabee over Romney four years ago, 40 percent to 15 percent. This time, Calhoun turned out for Santorum, too, but more narrowly: Santorum received 30 percent to 17 percent for Paul and 16 percent for Romney. But two other conservatives, Rick Perry at 16 percent and Newt Gingrich at 12 percent, cut into Santorum’s margin.

That said, an outright Santorum win would be all the more impressive, given the obstacles he faced that Huckabee didn’t.

Ron Paul’s vote was something altogether different. He won overwhelmingly among the young, and brought young voters into the caucuses. Somewhere around a third of Paul’s voters were under 30, compared with only 15 percent of all caucus-goers. Nearly a quarter of caucus-goers said they were independents, not Republicans, and they gave close to half their votes to Paul. His vast improvement over his 2008 showing — he appears to have doubled his vote — was built in large part on the votes of non-Republicans, or at least of voters who hadn’t thought of themselves as Republicans before.

For months, pundits of all political stripes went on and on about Romney’s difficulties with the right end of the Republican Party, including the overlapping constituencies of white evangelicals and Tea Party supporters. Whether Romney wins Iowa, or comes in second or third, it’s a problem he still has to deal with.