Musing on his presidential odyssey, Obama said Americans 'don't want spin, they don't want PR, they want straight talk.' -- PHOTO: REUTERS

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New Yorkers kicked off the unique nationwide primary contest before dawn, with a huge turnout expected, especially among energized Democrats, before last polls close in California on Tuesday night (0400 GMT Wednesday).

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 parched Arizona in the southwest.

It is the toughest test yet in the most expensive, intense, prolonged and unpredictable White House race, which will see Democrats eventually break a deadlock and pick the first black or woman presidential nominee.

But barring a major surprise, even 'Super Tuesday', the biggest one-day nominating bonanza ever, is unlikely to install Clinton or Obama just yet: their state-by-state race could drag on until March or even longer.

The two rivals duelled with campaign rallies and television advertising blitzes across the political map on Monday, with the race narrowing in Clinton's stronghold, California, tight in heartland Missouri and Tennessee, and up for grabs in states like northeastern New Jersey, Connecticut and Massachusetts.

VIDEO Polls go to the polls (1:32)

'I don't believe that you can rest until the last vote is counted, and so I'm going to keep working, reaching out to the voters,' Clinton told CBS News late on Monday.

The former first lady was to vote early on Tuesday in her home state of New York, while Obama was returning to his patch in Chicago to watch results roll in.

'I think we're bringing a lot of new voters into the process and we'll see a split decision, basically, coming out of tomorrow, with both of us having won sizable numbers of delegates,' Obama said on the same CBS program.

'I think we'll have to continue on,' he said.

Monday, her voice raw and fatigue creasing her face, the former first lady brushed away a tear as she visited Yale University, where her political odyssey started as an earnest 1970s student in bell-bottom pants.

Clinton, 60, held an online town hall meeting also broadcast on a cable television channel, taking questions from across 'Super Tuesday' states.

'It's going to take someone with experience in running and winning campaigns to take the White House in November,' Clinton said.

On talk show host David Letterman's couch, she said she, not former president Bill Clinton, would be the boss if she wins November's election.

'In my White House we will know who wears the pant suits,' she quipped.

To wild chants of 'O-ba-ma' and 'We can't wait,' the 46-year-old Illinois senator rocked an indoor arena packed with 16,000 supporters in the closely-fought state of Connecticut, then repeated the trick in Boston.

Musing on his presidential odyssey, Obama said Americans 'don't want spin, they don't want PR, they want straight talk.' A clutch of polls Monday showed the Democratic race a virtual dead-heat.

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.

The cliffhanger Democratic race contrasted with signs that McCain, a Vietnam war hero, would all but settle the Republican nominating fight on 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, who hoped late polls showing him moving in California would stall McCain's momentum.

A USA Today national 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, a former Massachusetts governor, 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 in Nashville, Tennessee.

'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 Democratic contests in 22 states, Republican contests in 21 states, and 19 states are holding nominating clashes for both parties.

Democrats are also hosting two additional primaries starting Tuesday: one in American Samoa and one for 'Democrats abroad'.

Voting for expat Democrats is taking place across the world until February 12, as well as online.

In Jakarta, where Obama spent part of his youth, Democrats handed him a win over Clinton in the first result announced, party officials said.

Seventy-five percent of nearly 100 votes cast by expatriate Americans went to Obama and 25 per cent went to Clinton, according to Democrats Abroad officials in the Indonesian capital. --AFP