Americans trooped to the polls Tuesday in key midterm elections, with Republicans expecting to claim the Senate majority on a day of reckoning for Democrats weighed down by an unpopular president.
Although many battlegrounds may go down to the wire, Democrats could lose Senate seats in as many as 10 states, a result that would hamstring Barack Obama in his final two years as president.
Polls suggest Republicans are on course to win the six extra seats they would need to gain control of both chambers of Congress for the first time since 2006.
And the party of an incumbent president historically fares badly in elections in the middle of his second term.
Every president since Ronald Reagan in the 1980s has left office with the opposition party controlling Congress, and Obama, following the costliest-ever midterm, estimated at $4 billion, is likely to be no different.
Many Republicans have essentially based their campaigns on attacks against the president and his policies like health care reform.
Much media attention has focused on recent crises like Ebola and advances by the Islamic State jihadists in Syria and Iraq, but polls suggest voters' minds were made up months ago.
Republicans have successfully capitalized on Obama's unpopularity.
"I think we need a change in American politics," construction worker Charles Kaster told AFP outside a polling station in Berryville, Virginia.
"We've given the other side six years to have their way with things and it doesn't seem to be working out too good, so I think it's time to switch back to somebody else."
Democrats hold a 55 to 45 seat advantage in the Senate, while Republicans control the House of Representatives.
Republicans, whom several top forecasters give about a three in four chance of winning the Senate, expressed confidence in the home stretch.
"Victory is in the air, we're going to bring it home tomorrow night!" ebullient Senator Mitch McConnell, the top Republican and potential new Senate majority leader, told a crowd in Kentucky Monday.
Republicans have hammered home their message that a vote for Democrats is a vote for a tarnished Obama and his policies.
In the House, where all 435 seats are in play, experts predict the Republicans will gain more seats. Thirty-six of 100 Senate seats are up for grabs.
Voters will also elect dozens of state governors and hundreds of local legislators, and decide on ballot initiatives including marijuana legalization.
However successful the Republicans are, a complete picture may not emerge Tuesday.
There are strong prospects for runoffs in Louisiana and Georgia, where rules require a second round if winners do not earn more than 50 percent of the vote.
Add to that a probable days-long ballot count in remote Alaska, where there is an unpredictable and tight race.
Louisiana's runoff is Dec. 6, but a Georgia runoff would be on Jan. 6, which means senators may not know who controls the chamber when Congress opens on Jan. 3.
Voters could get a sense of how the chips will fall nationally by keeping a close watch on two eastern states: North Carolina and New Hampshire.
Virtually all scenarios for Democrats maintaining Senate control rely on winning these two states, but experts say that if they flip Republican then Democrats will be in for a painful night.
Complicating matters, Kansas independent Greg Orman is neck and neck against veteran Republican Senator Pat Roberts.
Should Orman win he would have to choose which party he caucuses with, and he was giving no hints Tuesday.
"I'm not going there to represent the Democratic Party or the Republican Party. I'm going there to represent Kansas," Orman told CNN.
While both parties have rolled out their surrogates to rally voters, Obama, well aware of his status as lightning rod for Republican criticism, has largely steered clear of the campaign trail.
And he was laying low on Tuesday.
Vice President Joe Biden said he does not "agree with oddsmakers" and feels Democrats could hold the Senate, a sentiment the White House said Obama shared.