Washington ? Call it the law of unintended consequences.

A new national backup system meant to ensure that millions of eligible voters are not mistakenly turned away from the polls this year, as happened in 2000, could wind up causing Election Day problems as infamous as Florida’s hanging chads.

Congress required conditional, or provisional, voting as part of election fixes passed in 2002. For the first time, all states must offer a backup ballot to any voter whose name does not appear on the rolls when the voter comes to the polling place on Nov. 2. If the voter is later found eligible, the vote counts.

But Congress did not specify exactly how the provisional votes will be evaluated.

Add the ordinary problems that come with doing something new, and the result is a recipe for mix-ups at the polls and lawsuits over alleged unequal treatment of some voters, said Doug Chapin, executive director of Electionline.org, a nonpartisan clearinghouse for information on election reform.

“If I had to pick the one thing that will be source of controversy on Election Day, it will be provisional voting,” Chapin said.

State election officials have adopted their own and differing standards for when a provisional ballot will count; some of those rules are still in flux three weeks from the election.

Rules for who casts provisional ballots and how they are counted probably will vary even within states, especially if there are long lines, confusion and hot tempers at the polls, election experts said.

Some of the states where the race is tightest, such as Florida and Ohio, also have the strictest rules for provisional ballots.

Democrats and Republicans are training lawyers and election monitors to look for problems with provisional voting this year. Already, there are suits in five states claiming election officials are adopting too strict a standard for which votes will count and that eligible voters will be denied the right to vote as a result.

Questions about provisional ballots could produce a major battle after the election, too, with nightmarish echoes of the Florida fight of 2000.

Lawyers for President Bush and Democratic challenger John Kerry are ready for a new overtime contest in states where, if the election is close enough, the winner could be determined by who gets the most valid provisional votes.

Like Florida’s punch cards, provisional ballots are pieces of paper that must be evaluated individually and counted by hand. The task is time-consuming, and most states have short deadlines to get the job done, said Doug Lewis, director of the Election Center, a nonpartisan research and training organization for state and local election administrators.

Postelection suits could resemble the Supreme Court’s Bush v. Gore case that settled the 2000 election. The justices said it was unfair for Florida counties to apply different standards during punch card recounts, and there was not time to fix the problem.

Twenty-six states and the District of Columbia have adopted the view that a provisional ballot must be cast in the correct precinct, or it will not count.

Under that interpretation, voters unaware that their polling place has moved could be out of luck. So could voters given wrong information about their polling place. It would not matter whether the mistake was the voter’s fault or a clerical error.

Other states will count a voter’s choice for president and other national offices even if the ballot is not cast in the right local polling station. Votes for some purely local races might not count, but the theory goes that the voter should not lose out entirely just because of a ballot case in one precinct rather than another.

Provisional voting is not entirely new. About half the states offered something similar in 2000.

It is impossible to predict how many people will cast provisional ballots this year, said Curtis Gans, director of the nonpartisan Committee for the Study of the American Electorate.

It will easily be in the tens of thousands, however. In 2000 in Los Angeles County, the nation’s largest voting district, about 101,000 people voted provisionally. Of those, about 61,000 votes were determined to be valid.