Washington ? 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 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.”

Provisional ballots

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 mandated 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 provide 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.

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 precinct.

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.

Military ballots

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.

On Friday, in hotly contested Pennsylvania, Gov. Edward Rendell, a Democrat, 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 were 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.

Not everyone believes there will be a delay in announcing a winner on Tuesday, or even that the race will be particularly close.

Lawrence 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 argues, 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.”