A conservative candidate who hopes to win Iowa, like Mr. Cruz or Scott Walker, needs a substantial chunk of the evangelical vote. If Mr. Huckabee enters the race, he could pose a big roadblock to both. Even if he doesn’t win, he will make it easier for a relatively secular conservative candidate to win than has been the case in recent contests. And if Mr. Huckabee does win — as he very plausibly could over a strong, divided field — he will deny a more viable conservative candidate the easiest opportunity to consolidate the conservative opposition to Jeb Bush, or whoever wins New Hampshire.

And Mr. Huckabee, to be clear, is not an especially viable national candidate. He won virtually no support among non-evangelical voters in 2008, posting single-digit tallies among such voters in crucial states like Illinois and Florida. He did better among non-evangelicals later, but only after he was the only candidate remaining to challenge John McCain.

Mr. Huckabee suggested on Fox News that he could win the nomination if he won South Carolina, which went for Mr. McCain by just three points in 2008. But South Carolina wouldn’t make Mr. Huckabee a favorite to win the nomination. For a candidate trying to win with an evangelical coalition, capturing South Carolina — which was 65 percent evangelical in the 2012 Republican primaries — is not a big accomplishment. It’s a state he should win easily if he’s competitive nationwide. To earn the nomination, Mr. Huckabee would need to win far tougher states, like Illinois, California or Florida, where he barely eclipsed 10 percent of the vote eight years ago.