MANCHESTER N.H. — On the day Mitt Romney rolled out the endorsement of Senator John McCain of Arizona, a “super PAC” backing Newt Gingrich dug out an old gem and put in on the Web. The PAC, Winning Our Future, posted a commercial slamming Mr. Romney by using past footage of him offering conflicting positions on issues like abortion, gun control and what it means to be a Republican. “I will preserve and protect a woman’s right to choose and am devoted and dedicated to honoring my word in that regard,” he says. But moments later, in another clip, he...