So too with the Jeffords defection. To put it mildly, Bush won election as narrowly as anyone possibly could. He took office with a Senate divided precisely in half. Facing this reality, the Democratic leadership acted secretly and cunningly to woo Jeffords, a career-long misfit in the Republican Party. Call it the competence gap.
Parties gain and lose power because of shifts in the public's beliefs.
Reagan won over Jimmy Carter in 1980 because the public had become more conservative. But parties also gain and lose because of competence. Reagan won also because the Carter White House lacked competence. Right now the country is split down the middle ideologically and probably moving slightly in the Democrats' direction. That means competence will decide who will emerge as the majority party.