Today’s GOP primary voters will be less conservative and religious than Iowa caucusgoers
Although Iowa and New Hampshire both lead off the presidential nomination process and are both very white states, they have different-looking Republican electorates. Overall, New Hampshire GOP primary voters are less Republican, conservative and religious than Iowa Republican caucusgoers. This is due not only to differences across the two states' populations, but also divergent electoral rules that give New Hampshire a larger, less right-leaning electorate than Iowa. Thanks in part to these cleavages, the two states have voted for different winners in every open Republican presidential contest dating back to 1980.
One reason for these divergent results is that New Hampshire primary voters are less likely to identify as out-and-out Republicans than Iowa caucusgoers. Differing electoral rules help explain some of this contrast, as caucuses have lower participation rates than primaries. In Iowa, only registered members of a party may participate in that party's caucuses, and caucusgoers must attend these events at an appointed time on a weekday evening. By contrast, state-run primaries like New Hampshire's generally give voters roughly 12 hours to visit their regular polling place to cast a ballot, and they usually include at least a limited option to vote absentee. But critically, New Hampshire permits independent voters to participate in party primaries, unlike Iowa's caucuses and many other states' primaries.
New Hampshire Republican primary voters also tend to be less ideologically conservative than Iowa GOP caucusgoers. Once again, electoral rules play a role in this contrast. In part because of their higher barriers to participation, caucuses tend to attract the most committed and ideologically minded members of a party, whereas primaries attract a broader pool of voters, especially if independents are allowed to participate. Another reason for this ideological split is that New Hampshire's population has a smaller share of white evangelical Christians — a core part of the GOP base — than Iowa does. This is notable because white evangelical Christians are among the Americans most likely to identify as Republican, and they are more conservative than non-evangelical Republicans.
—Geoffrey Skelley, 538