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 toward explaining why Arizona Sen. John McCain hopes to take control of the race for the Republican presidential nomination in Super Tuesday's primaries and caucuses.

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

"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 wasn't always that way, but a change designed to weaken control of party bosses was ushered in after the riotous Vietnam War-era 1968 convention.

QUICK AND NOT-SO-QUICK

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 GOP's winner-take-all contests make a difference.

McCain, former Massachusetts Gov. Mitt Romney, former Arkansas Gov. 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. Endorsements from numerous party leaders soon followed, including Govs. 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.

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

McCain is favored 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 favorite 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 percent of the popular vote in a congressional district generally is guaranteed at least one delegate.

DIFFICULT TO PREDICT

In a race with two equally matched rivals — Obama and Clinton are both running well-funded national campaigns — 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 he or she wins 15 percent 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 percent of the vote to get a 3-1 split in delegates, and 85 percent 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.