Senate Democrats are hoping to hold on to a slim majority as the first results from the day's 37 races begin to emerge.
Whether Republicans can pick up the 10 seats they need to take over the Senate is a drama that could extend into the early morning, when results from several tight West Coast races may become clear.
And the Democrats' second biggest question -- the fate of the party's majority leader, Harry Reid of Nevada -- remains in doubt amid a tough challenge from Republican Tea Party favorite Sharron Angle.
So far, the evening's early results have not delivered any major surprises.
South Carolina Republican Jim DeMint, who championed a bold slate of Tea Party candidates, won an easy victory. So did Indiana Republican and former Sen. Dan Coats, who prevailed over Democratic Rep. Brad Ellsworth. Easily coasting to re-election was incumbent Democrat Patrick Leahy of Vermont.
In Kentucky, Republican Rand Paul, a Tea Party favorite, defeated Democrat Jack Conway, after a long-running war of words. Paul, the son of veteran Texas Rep. Ron Paul, is competing for the seat held by retiring GOP Sen. Jim Bunning. It's been a contentious race. At one point a former volunteer for Paul was accused of assault for stomping on the head of a left-leaning activist. And Conway supporters ran ads highlighting a college prank by Paul involving the "Aqua Buddha" and questioning his religious bona fides.
Among the most closely watched races:
Illinois -- In a race characterized by attack ads and finger-pointing, Republican Rep. Mark Kirk held a narrow lead in the polls against Democrat (and friend of President Obama) Alexi Giannoulias. Kirk drew fire for overstating his military record, while Giannoulias' family ties to a bank that dealt with disreputable characters became a focus of negative ads.
Colorado -- Incumbent Sen. Michael Bennet (D) is fighting for his political life against Tea Party Republican and former District Attorney Ken Buck. Buck wants to repeal the Obama administration's signature health care law and opposes abortion rights.
Washington -- Three-term incumbent Patty Murray is clinging to her seat in a contest against Republican Dino Rossi.
West Virginia -- Popular Democratic Gov. Joe Manchin has been running away from the Obama White House in a bid to hold on to a Senate seat held for decades by the late Democrat Robert Byrd. But Republican John Raese, as well as Sarah Palin -- who campaigned for him -- did their best to yoke Manchin and Obama. The victor will serve the remaining two years of Byrd's term. Manchin's former aide is under investigation by the FBI and Justice Department for alleged contracting abuses. Raese is under fire for an ad that featured "hicky"-looking actors.
Pennsylvania -- Former Club for Growth president and onetime GOP House member Pat Toomey is hoping to turn the Senate seat Republican after incumbent Democrat Arlen Specter lost in the primary. But Democratic Rep. Joe Sestak, a Navy veteran, has been campaigning nonstop to avert a GOP win.
Alaska -- Republican Joe Miller, a Tea Party favorite, is trying to hold on to an early lead in the polls over Democrat Scott McAdams. Incumbent Lisa Murkowski, who lost in the GOP primary, has launched a bold write-in bid to retain her seat and maintain her family's political dynasty in the state. If Murkowski is successful, she'd be the first write-in candidate to win since the late Strom Thurmond of South Carolina in 1954.
California -- Incumbent Sen. Barbara Boxer is trying to pull out a Democratic victory in a race against former Hewlett-Packard Chief Executive and John McCain presidential adviser Carly Fiorina.
Wisconsin -- Republican Ron Johnson, an Oshkosh plastics executive, is trying to defeat Democrat Russ Feingold, a three-term champion of campaign finance reform. Feingold voted for the health care bill and did not run away from that vote, unlike many struggling Democrats this year.
Florida -- Marco Rubio seems to be in line to win the seat for the GOP over Democrat Kendrick Meek and independent Charlie Crist. Crist, the incumbent governor and former Republican, is doing his best to make it a close contest. Copyright 2010 National Public Radio. To see more, visit http://www.npr.org/. Copyright 2010 National Public Radio. To see more, visit http://www.npr.org/. Copyright 2010 National Public Radio. To see more, visit http://www.npr.org/.