Voters grappled with partisan challenges to their registrations, broken equipment and other troubles Tuesday as legions of lawyers, election-rights activists and computer scientists watched for signs of voter disenfranchisement.
Election officials in Iowa, Michigan and Minnesota fielded complaints of disruptions by the liberal interest group MoveOn.org, while in Ohio, a woman sued on behalf of people who did not receive absentee ballots on time, asking that they be allowed to cast provisional ballots.
In Philadelphia, Republican activists claimed voting machines already had thousands of votes recorded on them when the polls opened. But city officials countered that the activists misunderstood numbers on odometers that records every vote ever cast - not just those for this election.
Nearly one in three voters nationwide, including about half of those in Florida, were expected to cast ballots using ATM-style voting machines that computer scientists have criticized for their potential for software glitches, hacking and malfunctioning.
Other major concerns were over provisional ballots, new this presidential election and a potential source of delayed counts, and whether poll workers were adequate and sufficiently trained.
"To a certain extent, provisional ballots are second-class votes," said Spencer Overton, a law professor at George Washington University. "You can cast a provisional ballot but we don't know if officials will count it."
Long lines greeted voters in many big cities in closely contested states, and some polls opened late.
At one New Orleans precinct, all three voting machines were broken and voters were told to come back later, said Bill Quigley, an attorney working for the NAACP.
In South Carolina, problems were reported in a handful of precincts in two counties using electronic machines. Officials said voters were forced to switch to paper ballots while technicians got the iVotronic touch screens from Electronic Systems & Software up and running within about 90 minutes.
Voters in one Richmond, Va., precinct using an old-style machine briefly cast ballots in the wrong congressional race.
And in Volusia County, Fla., a memory card in an optical-scan voting machine failed Monday at an early voting site and didn't count 13,000 ballots. Officials planned to feed the ballots, in which voters fill in a bubble, and count them Tuesday.
Tension was high at some Ohio polling places, including at one in Cleveland where a Democratic official claimed he was thrown out by a screaming poll judge before another told him he could return to the church basement.
Chellie Pingree, president of the citizens lobbying group Common Cause, said her group was running a toll-free voting complaint and information hotline that logged 20,000 calls by 10 a.m. EST.
"Many of the states where the election is closest and contested is where we're hearing from people most," she said.
Pingree said high turnout meant "more confusion to already overburdened, understaffed polling places, many of which will have as many lawyers and poll challengers as they have people voting."
A separate Web site and phone hotline maintained by nonpartisan and liberal voting-rights activists fielded thousands of complaints, including from people who showed up at polling stations to discover they weren't registered.
"If people are not being formally denied the right to vote, they are having to work hard enough to vote that many of them will not have the opportunity," said Will Doherty, executive director for the Verified Voting Foundation, a nonpartisan voting-rights group.
Both parties had thousands of lawyers dispatched and on call to respond to trouble. In a decision early Tuesday, a federal appeals court cleared the way for political parties to challenge voters' eligibility at polling places throughout Ohio.
A key problem is the lack of a unified voting system for the nation, the legacy of a patchwork of balloting technologies, regulations, partisan bickering and litigation.
A federal law passed in response to the 2000 election mess required states to offer provisional, or backup, ballots to voters who find they are not listed on the rolls, or whose eligibility is somehow in question. The ballots are set aside and evaluated after the election - they could take 10 days or longer to resolve.
But states have interpreted the law differently. Millions of newly registered voters may wrongly assume they can vote at any precinct in their city, town or county. State officials and courts have disagreed on whether provisional ballots are valid when a voter is at the wrong precinct.
The measure also requires first-time voters who registered by mail to provide identification when they show up at the polls, though disputes have arisen over whether to extend that to all first-time registrants and what documents count.
Add to that confusion: absentee ballots.
More than a dozen states missed the recommended deadline to mail ballots overseas, and in Florida's Broward County, thousands of absentee ballots went missing or got delayed.
As for electronic voting, many of the problems - whether accidental or intentional - may not be known until well after Tuesday - if at all. Most of the ATM-style machines, including all of Florida's, lack paper records that could be used to verify the electronic results in a recount.
Florida requires state election administrators to count - and, if necessary, recount - an election within 11 days. But lawsuits could drag out the results for weeks, even forcing the courts to decide the outcome.
Four years ago, the Supreme Court intervened in a recount after 36 days, handing George W. Bush a 537-vote victory in Florida and with it the presidency.