"If you have a real easy time of it, your campaign machinery may not be ready for the real campaign," he said.

While Bush was clearly challenged by McCain, Bradley's impact on Gore is less clear.

After winning narrowly in New Hampshire, Gore has coasted for the last few weeks. Meanwhile, Bradley's support consistently dwindled, and Bradley himself has backed off on his attacks, perhaps realizing that his chances for winning the nomination were virtually nonexistent.

Although Gore has not been tested since early February, White says Bradley offered enough competition before the New Hampshire primary to sufficiently challenge Gore.

"Bradley definitely made Al Gore a better candidate," White said.

On the other hand, a primary that is too contentious might hurt the eventual nominee. In Bush's case, McCain's frequent attacks against him means he can clearly emerge from the Republican primaries battle-tested, but he may appear vulnerable as well.

"It's always a problem when a party is bitterly divided during the primary season," Walt said. "Whoever wins has accumulated so much baggage that they become a weaker candidate in the general election."

Above all, the negative campaigning between Bush and McCain likely helped Gore most. Gore enjoys the benefits of negative attacks against Bush without appearing as a negative campaigner.