Washington is not in an area normally vulnerable to earthquakes, but on Tuesday, the earth began to shake all over town. The epicenter of the quake was up on Capitol Hill; specifically, in Suite 728 of the Hart Senate Office Building, the office of Sen. James Jeffords, the junior senator from Vermont. He was making a decision that would shift the Capitol landscape in a dramatic and substantial way.

Jeffords announced Thursday that he was leaving the Republican Party and declaring himself an independent. That would be news anytime, but with the Senate membership standing dead even at 50-50 and Vice President Richard Cheney casting the decisive vote to give the Republicans control, his switch changed the balance of power and gave the Democrats control. Senators have changed parties on occasion in the past, but never before has a sitting committee chairman switched parties, and never before has a senator switched to change the balance of power in the Senate.

Jeffords is a product of the long tradition of moderate and independent minded Republicans. That species is among the most endangered. With Jeffords defection, their numbers in national office is shrunk to just a handful. He found himself regularly and strongly at odds with the party leadership in the Senate and with the policies of President George W. Bush. His decision was one of conscience, but the ramifications of his decision are of enormous political moment.