Updated: 11: 27 p.m. ET - Democrats will continue to hold the majority in the Senate, CBS News projects.
Republicans needed to pick up four seats in order to overtake the slim Democratic majority, but even with several races outstanding, Democrats have passed the 50-seat mark.
A year ago, Republicans were reasonably hopeful of regaining control of the chamber. But in the final days of the campaign season, amid a handful of unforced errors by Republican Senate candidates, it started to look increasingly out of reach.
CBS News also projects Democrat Tammy Baldwin to win the Wisconsin Senate race, making her the first openly gay woman to be elected to the Senate.
10:57 p.m. ET - Even if Mitt Romney doesn't win the presidency, his running mate will still have a job to return to: According to CBS News projections, Republican Rep. Paul Ryan will hold on to his Wisconsin House seat.
10:22 p.m. ET - CBS News projects Democrat Tim Kaine will defeat Republican George Allen in the Virginia Senate race.
Kaine, the Democratic former governor, and Allen, a former senator and governor, were running neck-and-neck for months in the costly and closely-watched contest. In recent weeks, however, Kaine appeared to have eked out a slim lead.
10:09 p.m. ET - Republican Deb Fischer will win the Senate race in Nebraska, CBS News projects.
10:00 p.m. ET - CBS News projects that Joe Donnelly, a Democrat, will beat Republican Richard Mourdock in the Indiana Senate race.
Mourdock, a Tea Party-backed conservative, defeated longtime moderate incumbent Dick Lugar in a surprise upset in the state's GOP primary earlier this year, and was locked in a tight race with Donnelly in recent weeks. The race seemed to take a turn toward Donnelly, however, after Mourdock suggested that pregnancies resulting from rapes were "something that God intended to happen." The win, a pick-up for Democrats, will make it tougher for Republicans to overtake Democrats in the Senate.
9:49 p.m. ET - CBS News projects Democratic Senator Sherrod Brown, of Ohio, will be re-elected to the Senate.
9:46 p.m. ET - Democrat Elizabeth Warren will beat incumbent Republican Scott Brown in the closely-watched Massachusetts Senate race, CBS News projects.
The race has been among the most costly and contentious of the cycle.
Warren, a liberal favorite and consumer advocate, pulled ahead of Brown in recent polls, but the race has remained tight throughout.
9:26 P.M. ET - Republicans will retain control of the House of Representatives, CBS News projects.
According to CBS News projections, Republicans have hit the magic number of 218 seats needed to maintain control. CBS News projections has Democrats winning 195 seats. There remain a number of outstanding races for which CBS News has yet to project a winner.
Democrats needed to pick up 25 Republican seats in tonight's election.
9:00 p.m. ET - The polls are now closed in 14 additional states, including Colorado, Arizona, Michigan, Minnesota, Nebraska, Texas, and Wisconsin. CBS News projects that Republican Ted Cruz, a rising Tea Party star, will win the Texas Senate race, and Wyoming incumbent Senator John Barrasso, a Republican. CBS News also projects wins for Democratic incumbent Senators Debbie Stabenow, of Michigan, Kirsten Gillibrand, of New York, and Amy Klobuchar, of Minnesota.
8:30 p.m. ET - CBS News projects that Democrat Chris Murphy will defeat Republican Linda McMahon in the Connecticut Senate race. McMahon, the co-founder of World Wrestling Entertainment Inc., also mounted an unsuccessful Senate bid against Democrat Richard Blumenthal in 2010.
In Virginia, according to the state Board of Elections, long lines at the polls have delayed the reporting of results.
8:00 p.m. ET- CBS News is projecting Angus King, an independent, to win the Maine Senate contest. He will replace Olympia Snowe, the longtime moderate Republican Senator from the state. CBS News also projects that Democratic incumbent Bill Nelson will beat Republican Connie Mack in the Florida Senate race.
Also in the Senate, CBS News projects wins for Tennessee Republican Bob Corker, Delaware Democrat Thomas Carper, Maryland Democrat Ben Cardin, and Rhode Island Democrat Sheldon Whitehouse.
7:30 p.m. ET - Polls have now closed in Ohio, North Carolina and Virginia. CBS News is projecting Democrat Joe Manchin as winner of the West Virginia Senate race.
7:00 p.m. ET - With polls now closed in six states, the first exit poll tallies are starting to trickle in. But the presidential election is not the only contest at stake: Republicans are fighting for control over the Senate, and Democrats are taking a stab at regaining the majority in the House.
As of 7:00 p.m. ET, polls had officially closed in Georgia, Indiana, Kentucky, South Carolina, Vermont and Virginia. CBS News is projecting Bernie Sanders, an independent who caucuses with Democrats, as the winner of the Vermont Senate race.
No call has yet been made in Virginia and Indiana, which are home to some of the most closely-watched Senate races in the country: In Virginia, the Democratic former governor Tim Kaine is running against Republican George Allen, a former senator and governor. In Indiana, meanwhile, Democrats have a critical - and unexpected - opportunity to pick up a seat amid recent controversy from the conservative Republican candidate Richard Mourdock. Mourdock defeated longtime Senator Richard Lugar, a moderate Republican, in the primary.