WASHINGTON: When it comes to presidential primaries, Democrats and Republicans play by different rules.

One party likes to share. The other, not so much.Which goes a long way towards explaining why Arizona Senator John McCain hopes to take control of the race for the Republican presidential nomination on Tuesday.

And why the busiest primary day in US presidential election history may merely intensify the contest between Democratic rivals Hillary Rodham Clinton and Barack Obama.

Voters in a total of 24 US states and American Samoa, as well as Democrats around the globe, will cast ballots on what has been dubbed Super Tuesday. They will take part in a hodgepodge of primaries, caucuses, a state party convention and even Internet voting by Democrats living overseas.

Clinton’s lead in national polls has narrowed with Obama gaining momentum after his victory in last weekend’s South Carolina primary and key endorsements from Senator Edward Kennedy and other high-profile Democrats.

“The delegate selection process is designed to keep the campaign going for as long as possible” among Democrats, said Howard Wolfson, communications director for Clinton’s campaign.

The Democratic rules provide for delegates to be awarded proportionately on the basis of the popular vote. It was not always that way, but a change designed to weaken the control of party bosses was ushered in after the riotous Vietnam War-era 1968 convention.

This year, Wolfson added, the calendar “was designed to pick a candidate as quickly as possible.”

Instead, the result, he said, is “this unbelievable, grueling sprint from the 26th of December to the 5th of February that will not result in a nominee being chosen.”

The sprint has been no less grueling for Republicans. But the Republicans’ winner-take-all contests make a difference.

McCain, former Massachusetts Governor Mitt Romney, former Arkansas Governor Mike Huckabee and others slogged through several contests, trading victories and dividing delegates.

Then came Florida, in which McCain pocketed all 57 delegates in a breakthrough triumph even though the Arizona senator only received 36 per cent of the vote. Romney was close behind with 31 per cent, but did not get a single delegate.

Endorsements of McCain from numerous party leaders soon followed, including Governors Arnold Schwarzenegger of California and Rick Perry of Texas.

“It gives you a chance to end the race earlier,” said Charlie Black, a strategist for McCain. “Theirs is going to drag out.”

In all, Democrats have primaries in 15 states and caucuses in seven states and American Samoa on Tuesday, with 1,681 delegates at stake.

Republicans hold 15 primaries, five caucuses and one state convention, and pick 1,023 delegates.

The goal in primaries and caucuses is to win delegates to the parties’ national conventions this summer. A total of 2,025 delegates are needed to secure the Democratic nomination; 1,191 are needed for the Republican nomination.

Nine of the Republican contests on Tuesday are winner-take-all.

McCain is favoured in primaries in five of them — his home state of Arizona, as well as Rudy Giuliani’s New York, and New Jersey, Connecticut and Delaware. That’s a total of 251 delegates. Losers get none, no matter how close they come.

Romney is the favourite in the primary in Utah, where a large Mormon population gives him an edge for a winner-take-all prize of 36 delegates. He also plans to attend the West Virginia convention on Tuesday in hopes of locking down all 18 delegates there.

Missouri, with 58 delegates, is a winner-take-all battleground among McCain, Romney and Huckabee.

A variation on winner-take-all awards three delegates to the popular vote winner in each congressional district.

California is an example, with 53 congressional districts, three delegates in each. The winner of the statewide vote picks up an additional bonus of 11 delegates.

In a few cases, Republicans award delegates proportionately based on the popular vote.

Unfortunately for Romney, his home state of Massachusetts is one of them. The state has 40 delegates, and an opportunistic McCain scheduled a weekend campaign stop in hopes of winning some.

By contrast, a Democratic candidate who gains 15 per cent of the popular vote in a congressional district generally is guaranteed at least one delegate.

The Democratic race has two equally matched rivals — Obama, the Illinois senator who is the first viable black presidential candidate, and Clinton, the New York senator and former first lady who is bidding to be the first female president. They both are running well-funded national campaigns and that tends to leave the winner of the popular vote with only a narrow delegate advantage over a loser who runs a strong race.

Multiply that across dozens of congressional districts — 53 in California — and predicting the winner of the delegate struggle is a virtual impossibility.

Then it gets harder.

For the Democrats, in a congressional district with three delegates, two go to the popular vote winner, and the loser gets the third as long as they win 15 per cent of the popular vote.

But in a congressional district with four delegates, the winner and loser in a two-way race are likely to divide the spoils evenly. The winner must receive nearly 63 per cent of the vote to get a 3-1 split in delegates, and 85 per cent of the vote to win all four.

Then there’s the winning by losing scenario.

This can happen in states that award an odd number of delegates in some districts and an even number in others.

Imagine a candidate loses the statewide vote narrowly, but manages to win the districts that have an odd number of delegates.

A version of that happened in Nevada earlier in the year.

There, Clinton had more supporters attend caucuses, but Obama won the delegate contest, 13-12.—AP