Millions of Americans across 24 states have been casting their votes in the biggest day so far in the race to choose presidential candidates.
Nearly half the country's states are voting on Super Tuesday
Overall, 42% of delegates to the two main parties' nominating conventions are being chosen on Super Tuesday.
Republican Senator John McCain hopes to knock out his last serious challenger, ex-Massachusetts governor Mitt Romney.
But the Democratic race between Senators Hillary Clinton and Barack Obama remains too close to call.
The BBC's North America editor, Justin Webb, says one thing is certain: any candidate who does badly today will be finished.
WHAT IS SUPER TUESDAY?
24 states holding simultaneous contests to help decide the Democratic and Republican presidential nominations
More than half of each party's delegates - who will choose the candidate - are up for grabs
Key states electing large numbers of delegates include California, New York and Illinois
The only state to announce a result so far - hours before the others - has been West Virginia, where Republican outsider Mike Huckabee took all 18 delegates on offer.
After what has been widely seen as an exciting campaign so far, turnout was expected to be high - though glitches with voting machines in Florida, long queues to show voter ID in Georgia, and bad weather in other states may have put some people off.
Both parties' presidential hopefuls made last-minute pushes for votes on the early morning TV shows.
Mrs Clinton, who cast her own vote at a New York school, told CNN: "There are a lot of people who worry that the president just doesn't pay attention. I want them to know that I get it and I'll be there for them if they're willing to go out and vote for me today."
Her croaky voice betrayed the punishing schedules all of the candidates have endured.
One of her campaign team told the BBC that they made some 12 million calls on Monday alone asking people to vote for her.
Meanwhile, Mr Obama told NBC: "The fact that we've made so much progress I think indicates that we've got the right message, and the question is are we going to be able to pull some states out."
For his part, Mr Romney asserted his conservative credentials, telling Fox News that voters would be saying to themselves: "If I want the conservative [candidate] I've gotta vote for Mitt Romney - he's the only one who's got a realistic chance of getting this nomination."
Mr McCain, who was out campaigning in New York, said he was cautiously optimistic that the next 24 hours would settle his party's race in his favour.
"I'm happy that we're doing as well as we are, but this could be a long night," he told NBC.
MAIN REPUBLICAN CANDIDATES
Mike Huckabee: Former Arkansas governor, social conservative, evangelical Christian
Mitt Romney: Former Massachusetts governor, business background, Mormon
John McCain: Front-runner, Arizona senator, former US Navy pilot and Vietnam prisoner-of-war
Latest national poll figures: McCain 48%, Romney 24%, Huckabee 16%
(source: Washington Post/ABC)
A national poll for the Washington Post and ABC showed Mr McCain well ahead of his rivals. The Arizona senator had 48% against Mr Romney's 24%, with former Arkansas governor Mike Huckabee and Texas congressman Ron Paul trailing far behind.
A Reuters/C-Span/Zogby poll gave Mr McCain double digit leads over Mr Romney in New York, New Jersey and Missouri, although the former Massachusetts governor was ahead 40% to 32% in California.
After the surprise victory for Mr Huckabee, an ordained Baptist minister, in West Virginia, Mr Romney's camp accused Mr McCain of cutting "a backroom deal".
With Mr Romney leading after the first round of voting in the state, Mr McCain's supporters switched en masse to Mr Huckabee, to deny Mr Romney a victory.
Since winning a comfortable victory in the Iowa caucuses, the first of the race, Mr Huckabee's campaign has faltered.
Nine of the states holding their Republican primaries, including big states such as New York and New Jersey, have a winner-takes-all system.
Whoever gets the most voters in those states is awarded all of their delegates to the party's convention, where the candidate who wins more than 1,191 votes becomes the nominee for the presidential election.
On the Democratic side, a national Washington Post/ABC poll on Monday showed Senator Clinton's lead over Senator Obama had narrowed to 4%.
MAIN DEMOCRATIC CANDIDATES
Barack Obama: Illinois senator, hoping to become first black US president
Hillary Clinton: New York senator, wife of former President Bill Clinton, hoping to become first woman president
Latest national poll figures: Clinton 47%, Obama 43%
(source: Washington Post/ABC)
Opinion polls suggest a tight Democratic race in many states, but in California a Reuters/C-SPAN/Zogby poll showed Mr Obama opening a 13-point lead over Mrs Clinton.
California - the most populous US state - has 370 delegates up for grabs. Some 2,025 are needed to win the Democratic Party ticket.
Their delegates are allocated along more complex, proportional lines that can vary from district to district, making it more difficult to predict when one of the two remaining candidates will cross the decisive delegate threshold.
Further complicating the nomination contest is the presence of what are known as "super delegates" - members of the party hierarchy who will make up about a fifth of all those attending the party convention in late August.
SUPER TUESDAY: KEY TIMES
1100 GMT: Voting began in Connecticut, New Jersey and New York
by 1930 GMT: W Virginia Republican result
Midnight GMT: Polls close in Georgia, result due soon after
0100 GMT: Voting ends in 11 primaries and caucuses, inc Illinois and Massachusetts
0130 GMT: Arkansas primary ends
0200 GMT: Polls close in seven states, inc N York and Arizona
0400 GMT: Polls close in California
They can choose whom to support at any point between now and then. In close presidential races, their votes have proved decisive, our correspondent adds.
In three states, only the Democratic Party is involved, and in two, only the Republican Party. In the other 19, which together account for nearly half the US population, both parties are in action.
Apart from West Virginia, which made its choice in a convention earlier in the day, Georgia is the first state to end voting at 2400 GMT. Voting in California ends at 0400 GMT.
In some states, voters have to be registered with a party to take part in the primaries. But more than half of the Super Tuesday states allow independents to vote.
In California and New Jersey, and some other states, these unaffiliated voters number in their millions.
You can watch live coverage of all the Super Tuesday results on the BBC News website from 0000 GMT.