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Democrats who fear that the presidential nominating race will run all the way to convention in Denver in August are dusting down rulebooks to find out exactly what they face.

To win the nomination, Barack Obama or Hillary Clinton need to win 2,025 delegates from a grand total of 4,049 available from 48 states.

The two states missing are Florida and Michigan — their delegates have been banned from the convention as punishment for moving their primaries forward to January. Mrs Clinton, who won both of those elections, is pressing hard for the ban to be lifted.

In a “brokered” convention, no candidate can reach a majority of delegates before backroom negotiations and multiple ballots have been held to determine the winner. The last Democratic nominee chosen in such circumstances was Adlai Stevenson in 1952.

More likely, 2008 could result in a “divided convention” where one candidate has more delegates than the other but not enough to win outright without the support of the so-called super-delegates. This is why both campaigns are already turning attention to these 796 national party officials, members of Congress, Governors, and former presidents and Capitol Hill leaders.

Super-delegates are not bound by the results of primaries or caucuses — and can change their minds until the last minute. Mrs Clinton and Mr Obama are lobbying them furiously, making telephone calls and sending e-mails to try to gain their support.

The number promising support to each is disputed. Media organisations routinely call super-delegates who have not publicly endorsed a candidate in an effort to learn where they stand. CNN estimates that Mrs Clinton has 193 to Mr Obama’s 106.

Super-delegates were introduced in 1984 when the Democrats changed their nomination process to include an Establishment bulwark that would protect front-runners, and ensure that dark-horse candidates did not run away with the nomination. It was the divided convention of 1968 when grass-roots anti-war activists railed bitterly about the nomination of Hubert Humphrey — that has caused the party’s current difficulties. Robert Kennedy, widely favoured to win the Democratic nomination, was assassinated just two months before the Chicago convention. Vice President Humphrey did not participate in any primary election contest, yet was able to secure the nomination with delegates from non-primary states.

A subsequent rule change ensured that delegates are awarded proportionately in each state, meaning closely matched candidates like Mr Obama and Mrs Clinton are unable to secure the breakthrough they need to win the nomination. A split convention makes magnificent political theatre, but it can be ruinous for the party.The last candidate to emerge from a brokered convention to win the presidency was the Republican Warren G. Harding in 1920.

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