If Mike Huckabee didn’t exist, John McCain would have had to invent him. First, Mr. Huckabee won the Iowa caucuses, sending Mitt Romney into a tailspin that gave Senator McCain the opening he needed to win New Hampshire. Mr. Huckabee then took enough votes from Mr. Romney in South Carolina and Florida to allow Senator McCain to win both states with scant support from conservatives. Finally, by winning a string of Southern states on Feb. 5, Mr. Huckabee stole Mr. Romney’s last chance to convince anti-McCain Republicans to rally behind his candidacy. In states that Mr. Huckabee did not win,...