Hillary Clinton and Barack Obama battle coast-to-coast on Tuesday and John McCain aims to put a choke-hold on the Republican White House ticket, in a 24-state nominating clash unique in US history.
As the hours ticked down to "Super Tuesday" Clinton, wracked by fatigue after punishing months on the campaign trail, and Obama, energized by polls showing him scything into her once-gaping lead, battled for wavering voters.
The 46-year-old Illinois senator rocked an indoor arena packed with 16,000 supporters, in the closely-fought state of Connecticut, sparking chants of "O-b-ama" and "We can't wait."
Musing on his presidential odyssey, Obama, leading a hope-fueled crusade for political change, said he had been convinced "the American people, they don't want spin, they don't want PR, they want straight talk."
After a clutch of single-state contests, "Super Tuesday" embraces millions of voters from across racial, religious, social and income barriers, in states as diverse as liberal Massachusetts and "deep south" conservative bastions.
It is the toughest test yet in the most expensive, intense, prolonged and unpredictable White House race ever, in which Democrats will eventually make history by picking the first black or woman presidential nominee.
The two rivals battled across the political map, with the race narrowing in a Clinton stronghold, California, tight in heartland Missouri, and up for grabs in the northeastern states of New Jersey, Connecticut and Massachusetts.
But barring a major surprise, even "Super Tuesday" is unlikely to install Clinton or Obama just yet: their close state-by-state race looks set to go on until at least March.
Earlier, her voice raw and fatigue creasing her face, the former first lady wiped a tear from her eye as she visited Yale University, where her political odyssey started as an earnest 1970s student in bell-bottom pants.
"Well I said I would not tear up, already we are not exactly on the path," said Clinton, 60, in an emotional moment.
The cliffhanger Democratic race contrasted with signs that McCain would all but settle the Republican nominating fight Tuesday, to complete one of the most staggering comebacks in recent US political history.
"I'm guardedly optimistic," the Arizona senator told reporters in Massachusetts, the home state of his top rival Mitt Romney.
A USA Today poll gave McCain a 42 per cent to 24 per cent lead over Romney, with former Arkansas governor Mike Huckabee on 18 per cent.
But Romney refused to admit defeat. "This is going to come down to a real battle and I think I'm going to win it," he said at a campaign stop in Nashville, Tennessee on Monday.
"Super Tuesday" states account for more than half the Democratic delegates and almost half of Republican delegates at party conventions in August and September, which formally nominate candidates for November's general election.
There are 22 Democratic contests and 21 on the Republican side, with 19 states hosting nominating clashes for both parties.
A clutch of new polls showed the Democratic race a neck-and-neck struggle between two rivals bidding for history, as the first woman or African-American presidential nominee.
Clinton clung to a 45-44 point lead in a USA Today/Gallup national poll. A CBS/New York Times poll had the race deadlocked at 41 per cent.
A significant battle was brewing in California, the biggest "Super Tuesday" prize, where Clinton has led for months, but Obama was drawing level, or even ahead in some polls.
A loss in California would be a hammer blow for the former first lady, though her campaign was banking on the fact that millions of people took advantage of early voting, before Obama's latest poll surge.
Clinton led the Illinois senator 53 to 39 per cent in her home state New York in a new Quinnipiac University poll. The same survey had the race narrowing in neighboring New Jersey with the former first lady leading 48 to 43 per cent.
Her campaign was not publicly rattled by the Obama surge.
"During this whole election the polls have been all over the map," Clinton spokesman Doug Hattaway told AFP. "There are battlegrounds stretching from Massachusetts to California, so it could be a real nail-biter."
The Obama campaign tried to downplay expectations.
"We fully expect Senator Clinton to earn more delegates on February 5, and also to win more states," said Obama campaign manager David Plouffe in a memo.
His aides believe a split of the spoils on Tuesday would put Obama in pole position for later contests in February and March. (AFP)
'Super Tuesday' beckons as Obama, Clinton go head-to-head