Faced with a surge in voter registrations leading up to Nov. 4, election officials across the country are bracing for long lines, equipment failures and confusion over polling procedures that could cost thousands the chance to cast a ballot.

The crush of voters will strain a system already in the midst of transformation, with jurisdictions introducing new machines and rules to avoid the catastrophe of the deadlocked 2000 election and the lingering controversy over the 2004 outcome. Even within the past few months, cities and counties have revamped their processes: Nine million voters, including many in the battleground states of Ohio, Florida and Colorado, will use equipment that has changed since March.

But the widespread changes meant to reassure the public have also increased the potential for trouble.

"You change systems and throw in lots of new voters, and you can plan to be up the proverbial creek," said Kimball Brace, president of Election Data Services, a consulting firm that has tracked the voting changes.

Since Congress passed the Help America Vote Act six years ago, $3 billion in federal funds has been spent to overhaul voting operations, much of it for new equipment. With touchscreen machines falling out of favor, an increasing number of the nation's voters -- just over half -- will use paper ballots, which will be read by optical scanners. That will produce a paper trail that can serve as a backup if questions arise over tallies.

For more than half of the states, this will be the first presidential election using statewide databases required by the 2002 law to improve the accuracy of voter rolls. When voters arrive at the polls, their information must match the list in order for them to receive a regular ballot. That could trigger contentious questions in places with particularly rigid rules on what constitutes a match.

Both campaigns have lined up teams of lawyers to challenge any irregularities, from registrations to polling place problems to vote counts.

And experts say the problems ahead will be formidable, even if they don't rise to the level of the Supreme Court challenge over the 2000 results.

"The voting process is going to be tested in a way it has not been in recent history," said Tova Wang, vice president for research at Common Cause, a government watchdog group.

Recent local primaries have offered warning signs.

In the District last week, initial tallies were inflated by thousands of votes, causing chaos that night, and officials have yet to explain the problem.

In Palm Beach County, Fla., more than 3,500 ballots went missing in an August primary, forcing workers to hunt through bins and leaving a judicial election still undecided.

That same day, equipment problems in two other Florida jurisdictions delayed results for hours.

Premier Election Solutions, the company that makes many of the nation's voting machines, last month acknowledged that software used in 34 states, including Virginia and Maryland, could cause votes to be dropped. The company, formerly called Diebold, said it has no fix for the problem now, but election officials can catch the errors and recover the votes through a routine process of double-checking electronic memory cards.

Any weak spots in the process in November, whether poorly trained poll workers, a confusing ballot design or faulty equipment, will be further stressed by turnout, including many first-time voters.

During this year's presidential primaries, the number of voters hit an eight-year high in 36 states, according to Electionline.org, which monitors electoral reforms as part of the Pew Center on the States.

Maryland election officials said Tuesday that they expect 250,000 new voters to register by next month's deadline. More than 280,000 Virginians have registered to vote since the beginning of the year.

In the battleground state of Nevada, there are 400,000 more voters registered than four years ago. More than 500,000 have registered in Indiana since the beginning of the year, prompting Secretary of State Todd Rokita to say this could be "the biggest Election Day in our nation's history in terms of turnout."

Federal officials estimate that 2 million poll workers will be needed to handle the turnout, twice 2004's number and a goal states are scrambling to meet.

New York City had hoped to muster more than its usual 30,000 poll workers, particularly to help voters with disabilities, but extra funds were not available, said Marcus Cederqvist, executive director of the city's Board of Elections. "We will have waits -- I'd guess an hour or maybe two -- but we like to see high turnouts," he said. "It's what we are here for, and let's hope voters keep it in perspective. It won't be like waiting for an iPhone overnight."

Because elections are managed at the local level -- more than 10,000 jurisdictions run voting operations -- there is plenty of opportunity for foul-ups, which can resound nationally.

"Nobody wants to be that county," said Rosemary Rodriguez, chairwoman of the U.S. Election Assistance Commission, created in 2002 to oversee and enforce nationwide election reform. But, she added, "the biggest fear I have is that elections officials don't heed what they saw in the primary and plan."

After a spate of Election Day problems in Ohio in 2004, when some voters waited in line more than five hours, Franklin County, which includes Columbus, has added poll workers, increased the number of voting machines by 50 percent and commissioned a study on where the machines should go.

Other jurisdictions, including elsewhere in Ohio and several counties in Virginia, are requiring more training of poll workers, from greeters who will walk lines to make sure voters are at the right site to supervisors who must be able to set up and test voting machines.

In Worcester, Mass., local election officials are trying to prepare for the bigger turnout by locating some polling places in four supermarkets, which have plenty of parking and are accessible to disabled voters.

But David Moon, program director for FairVote, a voting advocacy group that is surveying local operations, said that "very few county officials" in swing states "are creating rational plans" to put machines where they are most needed. As a result, he said, frustrated voters stuck in long lines could give up and go home without casting ballots -- the same thing that happened four years ago in many states.

The process could be complicated by the statewide registration databases, which have been coming online one by one since 2004. For 31 states, Nov. 4 will be the first test of the systems with the bigger turnout of a presidential election.

States have taken a variety of positions on what should be considered a match when it comes to nicknames, hyphenated names and married names. If the information doesn't match, voters can cast provisional ballots, but whether those will count in final tallies depends on local rules, which vary widely.

"If you have small glitches multiplied by thousands of voters, that means big problems that cost eligible voters their voice," said Daniel P. Tokaji, an election law specialist at Ohio State University's Moritz College of Law. The problems could be more acute with hyphenated Hispanic names or transposed Asian surnames, he said, "leaving certain groups disproportionately affected."

Registration rules have prompted bitter complaints and lawsuits in Missouri, New Mexico and other states, and could lead to challenges after the votes are counted. Voting rights advocates have protested an Arizona requirement that residents show proof of citizenship to register, which has been upheld by a federal judge.

Advocates also worry that the back-and-forth of legislative debates and court rulings on voter identification in numerous states could further confound poll workers, disenfranchising some voters.

As they approach November, some local officials say they have addressed problems that surfaced in this year's presidential primaries.

Touchscreen machines still will be in place in Horry County, S.C., which includes Myrtle Beach, but elections director Sandy Martin said she will avoid the programming error that forced the county to use backup paper ballots -- some votes were cast on yellow legal pads -- and delayed results for a day.

"Oh, my gosh, it was awful," Martin said.

In Contra Costa County, east of San Francisco, registrar Stephen Weir said he too learned from the primary. A fold in the absentee ballots forced him to spend nearly two weeks ironing, by hand, about 16,000 ballots to make them flat enough to feed into vote-counting machines.

"There were two lessons learned," he said. "Dump the fold. And the silk setting worked great."