Never in the history of Congress has a party lost control of either the House or the Senate because of a party defector. That's a somewhat polite way of saying that losing James Jeffords is a blunder of historic proportions by the Republicans.
Party switches are actually quite rare.
Only 15 members of Congress in the past 20 years have changed parties while holding office. The 2000 Republican National Convention in Philadelphia was a nauseating, nonstop festival of pseudo-inclusiveness and scripted, superficial diversity marketing. Sen. Jeffords said he parted ways with his president and party on choice, judicial nominations, education, defense policy, energy and the environment. There's not much left to bring someone under the big top. When Lowell Weicker, the former Connecticut governor and senator, quit the party, he said that moderates were no longer a wing of the Republican Party, just a feather.