Forget Monica — Hillary’s greatest rival may be Grampa Munster. Actor Al Lewis, 89, announced in Albany on Monday that he’s formed a exploratory committee to study the possibility of his running for the Senate seat being vacated by Sen. Daniel Patrick Moynihan, D-N.Y. This is the same seat for which Hillary Rodham Clinton and New York City Mayor Rudy Giuliani are vying. Lewis, a longtime political activist, is best known for playing Grampa on the ’60s sitcom “The Munsters.” (He also played Schnauzer on “Car 54, Where Are You?”) He said he doesn’t think much of his potential competition. “They’re both the same — Rudy Clinton is just the latest version of Tweedledee and Tweedledum.”