Tuesday's mid-term congressional elections will determine the new power balance in Washington. Republicans are expected to win big, causing headaches for President Barack Obama.
The first polls close at 2200GMT, 1800 on the East Coast (EDT), and results will start coming thick and fast an hour later.
Here are key themes and races to watch for as the drama unfolds.
They say victory is sweet, but some victories are sweeter than others. For Republicans, toppling Nevada's Harry Reid - the Senate Majority Leader who many of them revile almost as much as the president - would be the sweetest victory of all. And it is well within their reach.
The contest between Mr Reid and Republican candidate Sharron Angle, a Tea Party favourite, is the race to watch on Tuesday night.
If Mr Reid triumphs, it will in part be due to Ms Angle's weakness as a candidate, her gaffes and mistakes - like running a series of anti-immigration ads in a heavily Hispanic state.
The race is so close, there may be a long wait for the result to come through, after polls close at 0100GMT / 1900PDT. For clues, watch the Hispanic turnout across the country - if Latinos are voting in high numbers, that may be good news for Harry Reid.
Republicans also have their sights set on President Obama's former Senate seat in Illinois.
The battle there between state treasurer Alexi Giannoulias, a Democrat, and Republican Congressman Mark Kirk - complete with accusations of lies, corruption, and ties to the mob - is typical of Chicago's bruising political scene.
Going into polling day, the race was too close to bet on, but if Republicans win this traditionally Democratic spot expect them to celebrate it as a serious snub to Mr Obama.
Republicans also hoped to claim Vice-President Biden's former seat in Delaware, but in nominating Tea Party darling Christine O'Donnell, they may have clutched defeat from the jaws of victory.
Ms O'Donnell's oddball pronouncements on masturbation, witchcraft and human-mouse hybrids have virtually handed a once-tight contest to Democrat Chris Coons. However, Democrats are not taking anything for granted this year.
There was a time when Republicans also thought they might claim Hillary Clinton's old Senate seat in New York, but her successor Kirsten Gillibrand proved a tougher, more agile candidate than anticipated.
As far as symbolic victories go, will Ms Gillibrand's easy ride when compared with the dogfight in Illinois say anything about the relative standing of Mrs Clinton and Mr Obama? Probably not, but that won't stop some Hillary Clinton fans from gloating.
Tea Party Trophies
Last time America went to the polls, the Tea Party didn't even exist. In the last two years it has become a boisterous political force - but is it a force to be reckoned with? Tuesday will be the first real test of whether the movement has a bite to match its angry bark.
Ms Angle and Ms O'Donnell are two of the most high-profile Tea Party-backed candidates, but there are others who have an even better chance of winning.
In Colorado, Ken Buck is narrowly leading incumbent Democratic Senator Michael Bennet. The setting for Barack Obama's dramatic acceptance speech at the 2008 Democratic National Convention, Colorado has long been a swing state, but in recent years Democrats had hoped to make it reliably theirs.
Coloradoans voted in huge numbers for the president in 2008, but in today's political climate Mr Obama's endorsement of Mr Bennet may well have hindered, rather than helped him.
Mr Bennet is casting Mr Buck as an extremist who would outlaw birth control and privatise social security. For his part, Mr Buck has successfully positioned himself as a political outsider, although he angered some of his Tea Party supporters when he called them "dumb-asses" for questioning whether Mr Obama was born in America.
In Florida's Senate race, Tea Party candidate Marco Rubio surged to the front of the pack last year, spoiling once-popular Governor Charlie Crist's hopes for a smooth ride to the Senate seat.
Mr Crist went so far as to quit the Republican party rather than be defeated in a primary match-up against Mr Rubio. It will have all been for nought if Mr Rubio cruises to the easy victory the polls seem to indicate. But if the moderate Mr Crist makes an election night comeback, then the Democrats' prospects in the rest of the country will start looking brighter.
Rand Paul, the Republican Senate candidate in Kentucky also appears poised for victory on Tuesday. When Tea Party-backed Mr Paul was nominated over the Republican establishment's candidate, Democrats had harboured hopes of stealing this conservative seat away.
But the Democratic candidate Jack Conway made the rookie mistake of running a nasty attack ad, and sticking with it far beyond the point where it had become political poison. His poll numbers drooped, and so did Democratic hopes of winning the seat.
Money, Money, Money
The 2010 midterms have been historic in many ways, yet they may well be remembered for shattering spending records. But does money buy you love?
Not if you're Meg Whitman, the former eBay CEO and Republican candidate for governor of California. Ms Whitman has spent more than any non-presidential candidate in history - over $163m (£102m), mostly drawn from her personal fortune.
But former Governor Jerry Brown looks set to waltz his way back to the statehouse, even though he has been outspent 10 times over.
In Connecticut, Linda McMahon, the former CEO of World Wrestling Entertainment, is also learning this lesson the hard way. She's poured over $42m (£26m) of her own money into a state where only around one million people vote. And she probably won't win even though her opponent Richard Blumenthal was tarred by accusations that he lied about his military service, implying he served in Vietnam when he did not.
In the big-spending state of Florida, Republican gubernatorial candidate Rick Scott is taking home the prize for having the most free-flowing wallet - he has spent over $60m (£37m) from his own bank account.
But that might be the only prize he takes home. Mr Scott is locked in a neck-and-neck contest in with Democrat Alex Sink.
Mr Scott, Ms McMahon and Ms Whitman between them have spent over $250m (£156m) of their own cash. And Mr Scott is the only one who has much chance now of getting something to show for it.
Back in June, when women were winning primary contests left, right and centre, pundits dubbed one particular evening of results "ladies' night". But now some ladies don't seem to be faring so well.
Arkansas Senator Blanche Lincoln,a moderate Democrat, is steeling herself for a drubbing by Republican Congressman John Boozman. She has become the fall guy (or gal) for voter anger over bailouts, unemployment, healthcare reform, and climate change legislation, which is a hot topic in rural Arkansas.
In Washington state, Democrats were shocked when previously popular three-term Senator Patty Murray suddenly looked vulnerable. A quietly hard-working senator, Ms Murray hasn't done anything particularly controversial. But she didn't have to - she is a victim of anti-establishment fever.
Recent polls have Mrs Murray pulling away from her opponent, Dino Rossi, but the fact that this race was close at all is sobering news for Democrats. If she loses, it's likely to come as the culmination of an unexpectedly devastating blood-letting for the Democrats - and could be the blow that loses them control of the Senate.
California's Barbara Boxer is a senator that conservatives love to hate. She has become a symbol of every liberal, greenie, hippie trend they loathe.
So Republicans were thrilled when former Hewlett Packard CEO Carly Fiorina threw down a challenge for the seat But California's liberalism is more stubborn than many Republicans expected, and Mrs Fiorina, who swung recklessly to the right in a tough primary election, has found the going hard.
Amid a flurry of both savvy and vicious ads from both sides, Mrs Boxer enters election day with a slight edge. Mrs Fiorina has an outside chance of taking the seat, but regardless of the victor, one lady is going home empty-handed.
In Missouri, Democrats thought they had recruited a champion in Robin Carnahan, a candidate from a well-respected political family running against a former Republican whip in the House of Representatives, Roy Blunt. In a year when any connection with Washington is toxic, Mr Blunt could have found himself at a big disadvantage - but Ms Carnahan has failed to connect with voters and is trailing badly in the polls.
In Alaska, however, incumbent Senator Lisa Murkowski is poised for a remarkable comeback after defeat in the Republican primary by Joe Miller, the candidate endorsed by former governor Sarah Palin.
Although her name will not be printed on the ballot, Ms Murkowski launched a campaign urging voters to write it down by hand. Such write-in candidates rarely gather momentum, but astonishingly, Ms Murkowski has. She looks set to become the first senator since Strom Thurmond in 1954 to be elected in this way.
Ladies may also fare well in governorship races. Nikki Haley looks set to become the first female governor of South Carolina. Regardless of who wins Oklahoma's governor's race, that state will have its first female governor, as both candidates, Jari Askins and Mary Fallin, are women. And Democrat Alex Sink is locked in a too-close-to-call race which may see her emerge as Florida's first female governor.
In 2006, Democrats took control of Congress in a landslide election. They won conservative seats that had eluded them for decades. And now they look set to lose them again.
This trend is likely to be seen nationwide, starting in some of the earliest states to finish voting.
Tom Perriello of Virginia, John Boccieriof Ohio, Heath Shuler of North Carolina, Patrick Murphy of Pennsylvania and Betsy Markey of Colorado are just a few examples of Democrats in conservative House districts who are likely to lose their scalps.
If those representatives win, on the other hand, it could be a sign that the Democrats' losses won't be as dire as expected.
The same can be said of some tight Senate races. If Joe Manchin wins in West Virginia or Joe Sestak in Pennsylvania - not to mention liberal favourite Russ Feingold in Wisconsin - then things will be looking up for Democrats.
But if the night isn't going their way, even some long-established party giants might fall. Stalwart liberal congressmen Barney Frank of Massachusetts and Dennis Kucinich of Ohio are both facing more serious challenges than they have in years.