The voters finally get their first say in Iowa on Monday, and thank heaven. We’ll finally be able to count votes for the presidential nominations, rather than dissect polls that have been notably inaccurate in recent elections.

The Republican caucuses will test whether Donald Trump can translate his large crowds and celebrity into actual voters who will sit in a cold gymnasium for hours; whether the Christian right has the clout to deliver the victory that Ted Cruz has long predicted; or whether Marco Rubio’s recent momentum...