White House hopeful Newt Gingrich called the House Republican plan for Medicare "right-wing social engineering," injecting a discordant GOP voice into the party's efforts to reshape both entitlements and the broader budget debate.

In the same interview Sunday, on NBC's "Meet the Press," Mr. Gingrich backed a requirement that all Americans buy health insurance, complicating a Republican line of attack on President Barack Obama's health law.

The...