In a campaign that has been chiefly shaped by debates, Newt Gingrich demonstrated Saturday night why he tops the polls — and why he won’t easily be dethroned. Facing tough questions from his Republican rivals and debate moderators in Des Moines, Gingrich parried blow after blow without losing his cool or showing the much-discussed undisciplined side of his personality. It was the former House speaker’s first debate as the undisputed frontrunner and not only did he stand up to the fresh scrutiny, he reminded Republican primary voters of his command of the issues, his intellectual dexterity and his self-assured, forceful...