The history of primary challenges to sitting presidents isn’t good for either the challengers or the presidents. Former Nixon and Reagan White House official Pat Buchanan challenged President George H.W. Bush in 1992. He lost in the primaries, and Bush lost in the general. In 1980, Sen. Edward Kennedy won several primaries in his challenge to President Jimmy Carter, but not enough. Mr. Carter lost in November. And in 1976, Ronald Reagan challenged President Gerald Ford, coming closer than any modern challenger to besting the incumbent. But he came up short, and so did Ford in the general election. The only semi-recent time an intra-party challenge to a sitting president has worked was in 1968, when Sen. Eugene McCarthy’s strong showing in New Hampshire and Sen. Robert Kennedy’s entry into the race prompted President Lyndon Johnson to drop out. But that was before the advent of the modern primary system, and it didn’t work out for the Democrats in the general election either.