In an election season that already has taken more than its share of unlikely turns, few moments have seemed more improbable than the crowd scene Friday afternoon at a Chick-fil-A along a busy suburban thoroughfare here. At least 400 people jammed the restaurant, leaving those in the back straining to get even a glimpse of a man whose presidential candidacy had been left for dead not five months ago. Against the glassed-in playground stood Newt Gingrich, microphone in hand, promising: “If you’re with me, I think we will win the most decisive election in modern times, and I think we...