Jim Webb's win over incumbent Senator George Allen on Wednesday gave Democrats their 51st seat in the 100-seat Senate, an astonishing turnabout at the hands of voters.
Allen was the sixth Republican incumbent senator defeated in Tuesday's elections.
The Senate had teetered at 50 Democrats, 49 Republicans for most of Wednesday, with Virginia hanging in the balance.
Webb's victory ended Republican hopes of eking out a 50-50 split, with Dick Cheney, the vice president, wielding tie-breaking authority.
Earlier on Wednesday the Democrats had their victory confirmed in Montana, one of the two remaining poll results.
Jon Tester, the Democratic candidate, beat Conrad Burns, the incumbent Republican by fewer than 5,000 votes, according to US television networks.
Earlier Democrats had reclaimed the House of Representatives in the nation's midterm elections.