Legions of lawyers, party volunteers, paid activists and even foreign observers will descend on polling places across the country today in what promises to be the most heavily monitored presidential election in U.S. history.

The tens of millions of voters heading to the polls will face new election laws and, in many places, new voting machines -- aimed at remedying the problems that produced the bitterly disputed outcome four years ago. But many election officials are worried that some of the changes could instead ensure a repeat in a race as tightly contested as the one between President Bush and Sen. John F. Kerry.

An army of lawyers from both parties will be manning polls in battleground states, and the legal wrangling over ballot issues continued to rage hours before the polls were to open. In Ohio, the GOP's plans to mount an aggressive program challenging voters at the polls was upheld by a federal appellate court.

In other battleground states, Democrats have plans to challenge the challengers. Democratic officials in Philadelphia, for example, have threatened to file federal lawsuits against individual poll challengers who violate citizens' voting rights through harassment or intimidation.

Election officials also are fretting about the impact of provisional ballots, which are used by people whose names do not appear on voter rolls. Such ballots cannot be counted until after Election Day and, for the first time, are being mandated nationwide. Officials and observers also worry about voting machines, whether new and untested or old and problematic.

Given these and other problems, election directors in battleground states are girding for long lines, legal challenges and glitches that could leave the outcome in dispute for days or weeks.

"There will be several states where we will not know the winner on election night," predicted New Mexico Secretary of State Rebecca Vigil-Giron (D), president of the National Association of Secretaries of State. "If we do, great, I'll eat my words. But I don't think I will have to."

Jan Baran, a Republican election lawyer not involved in this year's campaign, said: "If it's close, we'll be right back to where we were four years ago. It may not be the same issues, or the states, but there will be plenty of problems for lawyers and campaigns to fight about."

Despite the problems, election officials across the country stress that they have gone to great lengths to ensure fair balloting. Poll workers have undergone more detailed training, registrars have been briefed on laws governing voting in their states, and voters across the country have received guides that spell out their rights and specify their polling places. A surge in early voting could take the strain off the polling system today.

Extra security is also in place, reflecting concerns about terrorism in the wake of the Sept. 11, 2001, attacks and deep suspicions on both sides that the other party is up to no good. In Lake County, Ohio, the sheriff's department is guarding voting machines with the same vigilance "as they do the nuclear power plant," according to election director Jan Clair.

Robert A. Pastor, a longtime international election observer who directs the Center for Democracy and Election Management at American University, predicted "a very chaotic election" that may be more akin to contests seen in developing countries.

And DeForest B. Soaries Jr., chairman of the U.S. Election Assistance Commission, worries that advocacy for the two candidates "is turning into acrimony and vitriol that is dangerous. Even if we have a mess, I'm hoping we have a civil mess," he said.

Here is a look at some of the key obstacles that election officials in the battleground states must overcome.

Challenges at the Polls

Both parties have long placed poll watchers in key precincts throughout the nation, but actual challenges to voter eligibility are relatively rare. That is expected to change today.

Efforts to pre-challenge voters in urban centers in Ohio, Wisconsin and Nevada have been thrown out by courts or election officials, but GOP officials plan to proceed with placing thousands of at-the-poll challengers across the country. Democrats argue with Republican tactics for rooting out alleged fraud -- such as mailing letters to see which are returned as undeliverable (proving nothing other than the person having moved) -- and complain that the efforts are aimed at minority voters likely to support Kerry.

U.S. District Judge Dickinson R. Debevoise of Newark ruled yesterday that the Republican National Committee violated a 1980s-era court order that prohibits it from singling out minority voters for anti-fraud initiatives. The judge ruled that Republicans cannot use a list of 23,000 registered voters to challenge people at the polls in Ohio. The list was based on returned mail sent by the Ohio GOP, which Debevoise said proves nothing, and he found that it targeted areas with large concentrations of minority voters.

The ruling was a political setback to Republicans, who were appealing it last night. But lawyers on both sides acknowledged that it has a limited practical effect because under the ruling, GOP challenges of Ohio voters can proceed as long as they are not based on the disputed mailing list.

A federal appellate court based in Cincinnati threw out two related rulings by lower courts that would have banned challenges in Ohio, and the state Supreme Court overturned another ruling that would have limited the number of challengers at each polling place.

The ferocity of such disputes in Ohio and elsewhere has many election officials on edge.

"I'm preparing for war," said Ion Sancho, election supervisor in Leon County, Fla. "I'm not worried about long lines, I'm worried about civil disobedience and riots. People, particularly African Americans who believe that they were systematically disenfranchised by the 2000 election, are simply not going to take this."

Registration Woes

Election officials overwhelmed by a record number of new voter registrations have been working round-the-clock processing them, but many people may show up to the polls to find that they are not listed on the rolls. In Florida and Ohio, Democrats unsuccessfully sued after state election officials instructed local election officials to reject new voter applicants who signed an oath but forgot to check a box attesting to the same.

Republicans are concerned that bloated registration rolls could lead to double-voting and other types of Election Day fraud. Florida Republicans charge that 14,000 ineligible felons are registered and more than 900 have already voted or have requested absentee ballots because of problems with a statewide list of felons who have lost their voting rights.

Adding to the potential for trouble is the fact that two critical swing states, Minnesota and Pennsylvania, have adopted statewide registration systems that are getting their first presidential election test.

Voters who registered through the mail and did not include acceptable identification must, for the first time nationally, produce identification at the polls. Some states have gone further than the new federal law requires, mandating that everyone show identification. In Milwaukee, more than 5,000 voters with questionable addresses may be required to show proof of residency as part of an agreement with state Republicans.

The new requirements have caused confusion in recent elections across the country, with some poll workers incorrectly applying the rules. That could be compounded by the Election Day crush and the Election Assistance Commission's belief that there is a shortage of poll workers in every battleground state except Florida.

Provisional Ballots

Election legislation passed by Congress in 2002 requires that any voter whose name does not appear on the rolls be given a "provisional ballot" that will be counted after Election Day if it can be determined that the voter was eligible.

But Congress left it to the states to determine when and how those ballots should be counted, a decision that has resulted in many lawsuits in recent months. Some states will count those ballots regardless of where they are cast, but 27 states will do so only if they are cast in a voter's proper precinct.

By law, provisional ballots cannot be tallied until after Election Day. In Ohio, election officials predict that a quarter-million such ballots will be cast -- far more than the 165,000 votes by which Bush won the state four years ago.

If the number of provisional ballots cast outnumbers the votes by which Bush or Kerry is ahead in one or more states upon which the presidential race hinges, legal analysts say, the brawl over whether and how to count them will make the pre-election litigation battles look like schoolyard skirmishes.

The Machinery

If all the other hurdles are overcome, there is still the matter of technology. One of the lessons learned from the Florida debacle four years ago was that the country's error-prone election equipment was unable to produce a clear winner in a razor-thin election.

Some states, including Florida, moved quickly to scrap the widely discredited punch-card machines that left officials struggling to divine voter intent from hanging chads; the percentage of registered voters using electronic voting machines has more than doubled in the past four years.

But about 32 million voters in 19 states will still use punch-card ballots.

Nowhere will the discredited machinery be more widespread than in Ohio, where 72 percent of voters will use punch cards. With its 20 electoral votes, the state is considered crucial by Bush and Kerry.

In Cuyahoga County, which includes Cleveland, poll workers have been given strict instructions to tell every voter to "check for hanging chads." But election officials there are also praying for intervention.

Election director Michael Vu recently gave candles to top managers, lighting one during a conference call last week with board attorneys who were monitoring several of the lawsuits to hit the state in the final days of the campaign.

On the front of the candle is "Our Lady of Fair Elections," pictured with a pile of ballots at her feet. On the back, a plea:

"I place my feeble vote in your hands that mine might be counted," it reads. "Ensure each chad is completely removed . . . and give me the patience to endure as many recounts as I must."

Staff writers Ceci Connolly in Philadelphia and Manuel Roig-Franzia in Miami contributed to this report.

Duval County officials examine signatures from disqualified ballots in Jacksonville, Fla., as partisan observers monitor the process. Some new voter applications in the state were rejected on a technicality, which Democrats challenged.