At first glance, the Iowa results seem to suggest Mr. Cruz could count on support from evangelicals the rest of the way. He won 33 percent of them, according to the exit polls, which could position him to fare well in the South, where evangelicals represent an even larger share of the primary electorate than they do in Iowa.

But Mr. Cruz mainly did well among evangelicals because they’re generally more conservative than nonevangelicals, not because he held an especially strong appeal among them. That’s very different from a candidate like Mr. Huckabee in 2008, who did well among evangelicals regardless of their political ideology.

Mr. Cruz lost the “somewhat conservative” evangelical vote to Marco Rubio, and fared poorly among “moderate” evangelicals — even if he did fare better than he did among nonevangelicals.

Mr. Cruz could get away with this in Iowa, where 49 percent of evangelical voters were “very conservative” and just 8 percent were “moderate,” according to exit polls. But it doesn’t work as well in the primary states, where evangelical voters are far more likely to identify as “moderate,” according to past exit polls.