New computerized registration rolls and litigation over new voter identification laws in states like Arizona, Georgia, Indiana and Missouri have left many poll workers and voters unclear about the rules, including whether they are in effect, as the courts have blocked many of the new laws.

“We’re expecting arguments at the polls in these states that will slow everything down and probably cause large numbers of legitimate voters to be turned away or to be forced to vote on provisional ballots,” said Barbara Burt, an elections reform director for Common Cause.

Meanwhile, votes in about half of the 45 most competitive Congressional races, including contests in Florida, Georgia and Indiana, will be cast on electronic machines that provide no independent means of verification.

“In a close race, a machine error in one precinct could leave the results in doubt and the losing candidates won’t be able to get a recount,” said Warren Stewart, policy director for VoteTrustUSA, an advocacy group that has criticized electronic voting.

Deborah L. Markowitz, president of the National Association of Secretaries of State, was less inclined to sound the alarm. She said that since it was not a presidential election year and many states had encouraged voting by mail, fewer people would turn up at the polls than in 2004.

With computerized registration rolls, Ms. Markowitz said, there will be far fewer people incorrectly excluded from the new databases compared with when registration rolls were on paper.

“There will be isolated incidents, there is no doubt about that,” she said. “But over all the system will move faster and with fewer problems.”