Evolving conventional wisdom holds that the growing string of senior Republican senators packing it in for retirement or a governor's office means gloom for the party in next year's midterm congressional elections, which are usually a time for an incumbent president's party to suffer. But, counterintuitively, that may not be the case. Bill Clinton remembers the Republican revolution of 1994 only too well -- and so does one of his top aides then, Rahm Emanuel, Barack Obama's incoming White House chief of staff. Indeed, Emanuel, who took over the House seat of someone named Rod Blagojevich in 2002, was one...