For more than a year the US presidential campaign involved a bulging cast of candidates vying for attention and momentum in a few early voting states.

That phase has ended. The field has been winnowed and the Democrats and Republicans each have clear, head-to-head battles between two contenders as they hurtle into next week's nationwide “Super Tuesday” contests, when more than 20 states vote.

It will be the biggest single day of presidential nominating contests in American history and will stretch the campaigns' organisational and financial resources like never before.

On the Democratic side, Hillary Clinton and Barack Obama face different challenges because of the peculiar way that party rules stipulate how delegates get awarded. Because they are so tightly matched in resources and votes, it has become a long slog - a chase for delegates - that is forcing campaigns to dust off rule books that have not been used in a generation.

They are even micro-targeting individual districts with uneven numbers of delegates because, in such an even race, the spare vote could be critical at the Denver convention in August.

To get the nomination, a candidate must win the support of 50 per cent plus one of the 4,049 convention delegates. That means the magic number is 2,025. Super Tuesday brings an avalanche of delegates - 1,681 - but that is not enough to give either Mrs Clinton or Mr Obama the nomination.

Unlike the Republican system, where most states award delegates under a winner-takes-all system, the Democrats award the states proportionally, on a congressional district-by-district basis. A candidate may win the popular vote in a state but end up with fewer delegates.

For the Republicans, Super Tuesday may be more decisive. In most states, a candidate needs to win by only one vote to take all its delegates. This favours John McCain, who is well ahead in California, New York, New Jersey and Illinois - the biggest prizes on offer. Mitt Romney must at least slow his rival's nationwide momentum enough to stay in the race.

Times Recommends

• Appalachian voters lean towards Obama
• Iraq: Battle for business begins
• 50 dead in seven days in Mexico's drugs war