All to play for in the biggest ever Super Tuesday
  • Key day for both parties as voters in more than 20 US states make their choice of candidates... as more than 20 states vote in the United States' Super Tuesday primary elections – the country's largest-ever such vote.

    Polls indicate a close finish between Barack Obama and the front-runner, Hillary Clinton, in the 22 Democratic state primaries, with the gap still closing.

    Meanwhile, Republicans are contemplating a different situation, with John McCain hoping his commanding lead over his conservative rival, Mitt Romney, will give him a knockout blow that effectively ends the race.

    Mr Obama polls 43 points to Mrs Clinton's 45 in CNN's Poll of Polls, as his numbers close with hers in a battle in which 1,618 delegates will be chosen for a summer convention, where 2,025 are needed for victory.

    Mr Obama's managers say a good showing today will provide momentum for further primary battles later this month.

    Mrs Clinton's campaign is, meanwhile, hoping her lock on the party machine will keep her ahead.

    The Republican race is dominated by an ebullient Mr McCain, who has begun pushing into the centre ground of American politics with an eye on the presidential race itself.

    Meanwhile, the conservative wing of the party is split between Mr Romney and the third-placed challenger, Mike Huckabee.

    All eyes are on California, biggest of all the states, with 370 delegates. For the Republicans, it is a must-win state for Mr Romney.

    Mr Obama trails Mrs Clinton there by two percentage points, while a third of voters say they have not made up their minds.

    The uncertainty over Tuesday's race comes in part because it has never happened before. Never have so many voters – 40 per cent of the total – voted in primary ballots on the same day.

    The Republicans have 1,023 delegates at stake. Mr McCain leads with 93 delegates, followed by Mr Romney with 77, Mr Huckabee at 40 and Texas Congressman Ron Paul with four, according to the latest tally.

    For Mr Obama and Mrs Clinton, the Democrats award their delegates proportionally and the two campaigns have said they do not expect either candidate to deal the knockout blow to secure the nomination from today's results.

    Originally, the state primary elections were spread across the calendar, allowing candidates to conduct "whistle-stop" tours of the nation.

    But in a race to be among the first states to poll, the original total of seven Super Tuesday states has swollen to 24.

    Privately, many campaign officials are furious that candidates cannot hope to dovetail their messages to individual states.