After four years of legislation, technology upgrades and other reforms aimed at avoiding a repeat of the hotly contested 2000 elections, a growing number of government officials and voting experts are preparing for the unthinkable.

Americans may not know who won the presidential race on Tuesday night. Again.

A surge in new voter registrations, coupled with widespread use of absentee and provisional ballots, could provide enough uncertainty in key states to force a delay in announcing a clear winner in the race between President Bush and Sen. John F. Kerry, according to many election officials and observers.

Legal battles in closely fought precincts could also cause or exacerbate delays, authorities said.

Several scenarios could produce an outcome in which the ballots that are not counted until after Election Day in crucial swing states are greater than the margin that separates the candidates on election night. If the state is considered crucial -- such as Ohio, Pennsylvania or Florida -- the result could be a delay of days or even weeks, officials and experts said.

"All I can say with certainty is that we will know who the next president is sometime between November 2 and January 20," said Doug Chapin, director of Electionline.org, a nonpartisan Web site focused on voting issues. "I'm not predicting chaos necessarily, but it's not unreasonable to think that it's a real possibility. . . . It will be entirely dependent on how close the election is and where it's close."

One of the biggest problems may be provisional ballots, which will be given to voters whose names do not appear on election rolls -- and which are being required for the first time nationwide by federal election reform legislation.

Hundreds of thousands of such ballots are expected to be cast nationally. But election officials cannot count those ballots until they have been reviewed for voter eligibility after Election Day. States have different deadlines for conducting those tallies.

In Ohio, officials are predicting nearly a quarter of a million provisional ballots, and local election boards have until Dec. 1 to count them all. In 2000, Bush won Ohio by 165,000 votes.

"If it's close, you are likely not to know the winner for a month," said Carlo Loparo, a spokesman for Ohio Secretary of State J. Kenneth Blackwell (R).

The special ballots caused delays in one race in 2002, setting back for five weeks the declaration of a winner in a Colorado congressional race.

Provisional ballots have also been at the heart of numerous litigation battles, and may prompt challenges once the polls are closed. The counting rules for provisional ballots vary; some states count them regardless of where they are cast, and others insist they be cast in the right precincts. This week, Iowa's attorney general said election officials will not count ballots cast in the wrong precincts on election night but will set them aside so that lawyers can fight over how to count them after Election Day, should the race depend on their outcome.

Another possible cause of delay is the counting of military ballots. A number of states, including the battlegrounds of Florida, Iowa, Colorado and Washington, will count military ballots that arrive after Election Day as long as they were postmarked on or before Nov. 2. Florida will count military ballots received through Nov. 12.

Yesterday, in hotly contested Pennsylvania, Gov. Edward G. Rendell (D) agreed to a seven-day extension to settle a federal lawsuit in that state after initially resisting the idea. As a result, military ballots there may be received through Nov. 10. Republican officials are considering a new lawsuit to push back that extension further.

Absentee ballots, the number of which has surged in the weeks leading up to Election Day, could be yet another cause of delay. More states than ever offer "no excuse" absentee voting, and both parties have strongly encouraged voters to take advantage of the option. But absentee ballots must be hand-counted starting on Election Day, a process that takes a significant amount of time.

DeForest B. Soaries Jr., chairman of the U.S. Election Assistance Commission, said "it's certainly possible" that no winner will be declared on Election Day, but he cautioned that any delay will not necessarily be the result of a dispute over the accuracy of the balloting.

"A delay in the conclusion does not in and of itself mean that the system has collapsed," said Soaries, whose commission was created to help administer federal elections under the Help America Vote Act of 2002. "It could mean quite the opposite -- that the system is working."

Not everyone believes there will be a delay in announcing a winner on Tuesday, or even that the race will be particularly close. Lawrence R. Jacobs, director of the 2004 Election Project at the University of Minnesota's Hubert H. Humphrey Institute, argues that both history and statistics indicate the race is unlikely to be so close that it results in delays or legal fights of the kind seen in 2000.

Only two of the last 10 presidential elections had total voting margins of less than 1 percent, Jacobs said, and even in those races the differences in close states were measured in thousands or tens of thousands of votes. Furthermore, Jacobs said, even if a state such as Ohio remains undecided, the odds are slim that it will matter to the overall race.

"Florida was a historical freak, an anomaly," Jacobs said. "We had a few hundred votes making a difference. . . . History suggests that is very unlikely to happen again."

Nonetheless, election officials and party lawyers are preparing for a long battle. The Kerry-Edwards campaign, for example, has chartered at least five planes that will be sitting on a runway, ready to carry lawyers to any remaining battlegrounds early Wednesday.

"We're hoping against all hope that this will be decided," said Mark Weaver, a lawyer for the Ohio GOP. "But every prudent election lawyer who watched Florida in 2000 is . . . hoping for the best and planning for the worst."

In New Mexico, which Democrat Al Gore won by 366 votes in 2000, election chief Rebecca Vigil-Giron is predicting that the state could have 30,000 provisional ballots cast. That would be nearly 100 times the margin by which the state's election was decided four years ago.

"I'm hopeful it won't be a problem, but I just don't know," Vigil-Giron said. "Every election official out there is just praying that whoever wins, wins by a big margin."