Over at The Ticket, David Chalian explains:

In 2008, 60 percent of the Republican caucusgoing electorate described themselves as born-again or evangelical Christians, and Mike Huckabee’s ability to win 46 percent of those voters propelled him to victory. The Perry, Bachmann, and Santorum campaigns are hoping that evangelical Christians will make up a similar proportion of the overall electorate tonight, too. If these voters account for significantly less than 60 percent of the electorate tonight, the Romney campaign is likely to see that as a very hopeful sign.
The two additional demographic indicators of most interest to the campaigns tonight are age and party identification. One overlooked statistic from 2008 is that Mike Huckabee trounced Ron Paul and Romney among younger voters. Paul and Romney drew an equal level of support from voters between the ages of 17 and 29 four years ago. (If you’re 17 but will turn 18 before Election Day in November, you can participate in the Iowa caucuses.) These caucusgoers, however, made up only 11 percent of the electorate. If the Paul revolution is to win its first caucus or primary on Tuesday night, the campaign will need to increase participation among 17- to 29-year-old voters.