We expect Iowa Republicans to have a very white, fairly religious and relatively conservative electorate. To start with, the state is very white. Among the 50 states, Iowa has the sixth-largest non-Hispanic white population share (84 percent). White voters make up a large part of the Republican base nationally, and as Iowa is whiter than most places, the GOP ends up having an almost entirely white electorate there: In 2016, for instance, the National Election Pool entrance poll found that 97 percent of Republican caucusgoers identified that way.
White evangelical Christians will also play a big part in the caucuses. After all, at least 56 percent of the electorate identified as such in the 2008, 2012 and 2016 GOP caucuses, per the entrance polls. Iowa's population as a whole is 19 percent white evangelical Protestant, according to the Public Religion Research Institute, which places it a bit above the national share of 14 percent. But white evangelical Christians are among the Americans most likely to identify as Republican, so we'd naturally expect them to be a more significant force in a GOP nomination contest. (We do have to be wary about the term "evangelical," which has become more inextricably linked to politics at the expense of its religious meaning.)
Lastly, there's good reason to expect a fairly conservative electorate. In 2016, the entrance poll found slightly more participants identified as "somewhat conservative" than "very conservative" — 45 percent to 40 percent — but more voters said they were very conservative in 2008 and 2012. In all three years, no more than 15 percent identified as "moderate." And generally, low-turnout caucuses tend to have more ideologically extreme participants than primaries.
This is in no small part due to higher barriers to participation. Only voters registered as Republicans can participate in party caucuses, which are a time-consuming, one-time event — unlike primaries where voters have all day to cast a ballot (and usually some option to vote by mail). In 2016, a combined 16 percent of Iowa's voting-eligible population took part in the Democratic and Republican caucuses, according to the U.S. Elections Project. This marked the highest turnout for any state using caucuses, but that figure was lower than the turnout in any state that held both a Democratic and Republican primary. Ultimately, the caucus system results in a low-turnout nominating event that magnifies the involvement of the party's most committed members.