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 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 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 GOP's 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. 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.

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

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