ONE OF THE MOST disturbing, even destabilizing, aspects of the presidential election is the prospect that partisans on either side will deem the outcome invalid if their candidate doesn't win. Republicans are warning of massive fraud by new registrants not entitled to vote, while Democrats complain of an organized program to intimidate and disenfranchise eligible voters. The tension between preventing fraud and ensuring access is inherent in every election. But the expected closeness of this one, combined with the lingering bruises of the 2000 race, the record number of newly enrolled voters and the added uncertainties posed by a new federal law, have ratcheted that inevitable conflict to a new level. Those factors have produced an avalanche of pre-election litigation, threaten a difficult and perhaps ugly Election Day in the closest states, and raise the possibility that the victor may once again not be known for weeks.

With millions of new voters added to the rolls, one of the battles from now through Nov. 2 will be whether those new registrations are valid. In Ohio, Republicans challenged 35,000 new voters who the party said may not have been properly registered because mail sent to their address had been returned; a judge blocked their effort to have those voters purged from the rolls, but that ruling is being appealed. In any event GOP poll-watchers still plan to challenge those voters if they turn up on Election Day. Precisely how such challenges will be conducted isn't clear, and they would vary from state to state, but it's easy to imagine huge Election Day delays as individual voters are judged eligible or not. Republicans are planning their most aggressive monitoring efforts in Democratic (and often minority) precincts. Fraudulent registrations shouldn't be tolerated, but absent good evidence that a particular voter is ineligible, the GOP ought to proceed with caution.

A related area of uncertainty stems, ironically, from a new law that was supposed to help fix the problems of the 2000 election. The Help America Vote Act requires that would-be voters who arrive at the polls to find that they are not on the rolls -- or whose registrations are challenged, as we described -- be given "provisional ballots." But as with the voter challenge procedure, some key states, including Ohio, lack clear standards and procedures for assessing the provisional ballots. The process of counting the provisional ballots could take days; Florida, for example, has no set deadline. And if counties use different standards, that could open the door to claimed equal-protection violations along the lines of those found by the Supreme Court in Bush v. Gore.

If that's not enough to make you nervous, consider this: In Ohio four years ago, when provisional ballots were used for a narrower purpose, about 100,000 such ballots were cast. The margin of difference between George W. Bush and Al Gore was about 165,000 votes. With polls showing a closer race this time, and with the use of provisional ballots apt to be more widespread, the country could be in for another long and unpleasant post-election period. Both candidates should weigh the potential costs to the country as they deploy their lawyers.