Senate minority leader Mitch McConnell has a theory about divided government. It's this: When one party holds the White House, and the other holds one or both houses of Congress, the chances of passing landmark legislation improve dramatically. McConnell cites two examples. The first is the passage in 1983 of a Social Security reform (and bailout) bill that brought President Reagan and Democratic House speaker Tip O'Neill together. The other came in 1996 when President Clinton signed a sweeping welfare reform bill drafted by a Republican Congress.A theory held by the leader of the minority party in Congress normally wouldn't...