(CNN) -- Democrats took control of the House of Representatives for the first time in a dozen years, but the crucial question of which party will run the Senate hung Wednesday morning on neck-to-neck contests in Montana and Virginia.
Democratic challengers have picked up four seats in the Senate, according to CNN projections. Republicans would need to take just one of the two remaining competitive races to keep control of the chamber.
Democratic Senate candidates won in Rhode Island, Pennsylvania, Missouri and Ohio, as well as independent Senate candidates Bernie Sanders in Vermont and Joe Lieberman in Connecticut -- who are expected to vote with the Democrats.
But in Montana, the race between Republican Sen. Conrad Burns and state Sen. Jon Tester is too close to call, although Tester shows a razor-thin lead. With 99 percent of the ballots counted, Tester leads by fewer than 2,000 votes.
If Tester wins, Democrats could secure Senate control by winning in Virginia, where embattled Republican Sen. George Allen trailed his Democratic challenger, Jim Webb, by fewer than 6,000 votes out of more than 2.3 million counted.
Webb declared victory early Wednesday, saying, "The votes are in, and we won."
But Allen wasn't backing down. "The election continues," he said.
If the parties split the Virginia and Montana races, that would create a 50-50 breakdown in the Senate, assuming that Sanders and Lieberman will caucus with the Democrats. In that event, Vice President Dick Cheney's constitutional authority to break tie votes would keep the Senate under Republican leadership.
GOP: 'Deeply disappointed'
House Majority Leader Rep. John Boehner, R-Ohio, said Republicans were "deeply disappointed in the outcome."
"Our challenge as Republicans is to regain our confidence, our courage and our energy to address the big issues that matter," Boehner said in a statement.
