The correct answer is Senator James F. Jeffords.
The United States Senator from Vermont that changed the balance of power in the Senate by leaving the Republican Party and becoming an independent is James Jeffords.
On May 3, 2001, Senator James F. Jeffords of the state of Vermont announced that he would leave the Republican Party. This caused major problems to the Republicans and President George W. Bush. The Republicans lost the majority status in Congress and President Bush legislation would not be easily passed because the Democrats could impede it.