WEST PALM BEACH, Fla. - Here in a county that knows a thing or two about Election Day meltdowns, both parties are fretting over what might go seriously wrong before, during or just after the Nov. 6 presidential election.

"More than 50 percent of the provisional ballots are thrown in the trash in this state," Florida state Rep. Mark Pafford told about 80 retirees who gathered for last week's meeting of the Golden Lakes Democratic Club.

That's only a slight exaggeration - 48 percent of the provisional ballots cast in Florida in 2008 were rejected. And Pafford's warning underscores anxiety in Florida and other states about legal challenges, ballot problems or bizarre outcomes that could bedevil a race that seems likely to be close - conceivably as close as the 2000 contest that people still quarrel about.

Merely the mention of that election unsettles people in Palm Beach County. The county's poorly designed "butterfly ballot" confused thousands of voters, arguably costing Democrat Al Gore the state, and thereby the presidency.

Gore won the national popular vote by more than a half-million ballots. But George W. Bush became president after the Supreme Court decided, 5-4, to halt further Florida recounts, more than a month after Election Day. Bush carried the state by 537 votes, enough for an Electoral College edge.

"Pregnant chad" entered the political lexicon. And Americans got a jolting reminder of the Founding Fathers' complex recipe for indirectly electing presidents.

Even if everything goes smoothly, it's conceivable the nation will awaken to a major shock in three weeks: an Electoral College tie between President Barack Obama and Republican Mitt Romney. That would throw the decision to the House of Representatives, currently controlled by Republicans but up for grabs in this election.

A 269-269 Electoral College tie is unlikely but far from impossible. It could result, for instance, if Romney wins all the competitive states except Ohio, Wisconsin and New Hampshire.

Four U.S. elections, including 2000, saw the presidency go to the person who finished second in the popular vote. There has never been an Electoral College tie. However, the U.S. House handed the 1824 election to John Quincy Adams after he finished second - in both the popular vote and the Electoral College vote - in a four-man race in which no one won a majority in either count.

An Electoral College tie isn't the only nightmare scenario that could raise doubts about the election's fairness and worsen partisan bitterness, which already divides Americans and makes compromise in Congress so difficult.

Campaign activists in many states are bracing for possible confusion, delays and even confrontations in polling places on Nov. 6. They are particularly watching Democratic-leaning precincts where Republicans may challenge some people's eligibility to vote.

In recent years, Republican officials in several states have pushed for tighter voter restrictions, including requirements for photo identifications and reductions in the amount of time allowed for early voting. Republicans say they are trying to prevent voter fraud. Democrats, however, note the absence of proven cases of serious election fraud. They say the GOP actions are meant to suppress voting by Democratic-leaning groups such as blacks, Hispanics, low-income people and college students.

Democrats have won court rulings in several states curtailing GOP efforts to shorten early voting periods and require new forms of identification. One Republican initiative that survived, however, is the end to a Florida tradition of allowing voting on the Sunday before Election Day, the "Souls to Polls" day when some black churches would urge congregants to vote upon leaving services.

The Obama campaign has amassed an army of lawyers and non-lawyer volunteers to watch voting places and quickly appeal to state and local election officials if they think legitimate voting is being impeded.

Since 2000, "we've had an amazing group of dedicated lawyers that have been on the ground for 12 years," said Charles Lichtman, a Fort Lauderdale-based attorney helping oversee the Democrats' effort. "So there's nothing they can throw at us that we haven't seen or that we're not ready for."

Other states are doing the same. A single memo seeking lawyers and law students to help safeguard Obama's voter turnout efforts netted nearly 4,000 responses, said Robert Bauer, the campaign's chief lawyer and a former White House counsel.