More than 200,000 registered Ohio voters may be blocked from casting regular ballots on Election Day because of a federal appeals court decision on Tuesday requiring the disclosure of lists of voters whose names did not match those on government databases, state election officials and voting experts said.

The court decision requires Jennifer Brunner, the Ohio secretary of state, to provide the names to local election officials by Friday. Once the local officials have the names, they may require these voters to cast provisional ballots rather than regular ones, and they may ask partisan poll workers to challenge these voters on Election Day. Both possibilities could cause widespread problems when the voters show up at the polls.

Concerns about those problems led the Ohio Attorney General’s Office to file an appeal of the decision to the United States Supreme Court on behalf of Ms. Brunner on Wednesday night.

The state’s appeal went directly to Justice John Paul Stevens because he oversees the Sixth Circuit. It argued that the Republican Party had nearly two years to raise complaints about the process of screening voter registrations and failed to do so. Any changes now to the process would disrupt preparations for the election, it contended.

Federal law requires states to verify voter registration applications with a government database like those for driver’s licenses or Social Security cards. Names that do not match are flagged for further verification. Since Democrats have been more aggressive at registering new voters this year, the decision will probably affect their party’s supporters disproportionately. Polling in the state shows Senator Barack Obama, the Democratic presidential nominee, with a slight lead on his Republican challenger, Senator John McCain.

Republicans have been angered by reports of voter-registration fraud linked to groups allied with Democrats, like Acorn, a community organizing group with ties to Mr. Obama. This month, the Ohio Republican Party filed a motion seeking to force Ms. Brunner, a Democrat, to hand over the list of all registration applications that had been flagged when checked using the state or federal databases.

In court papers, Republicans said they wanted the names to file challenges.

The 50-page decision by the Court of Appeals for the Sixth Circuit, in Cincinnati, reversed a 2-to-1 ruling by a three-judge panel of the same circuit last week.

It remains unclear when access to the lists would be given to local election officials. Before the state’s appeal to Justice Stevens, the appellate judges upheld a lower-court ruling giving Ms. Brunner until Friday to make the list available.

Social Security data indicate that Ohio election officials found more than 200,000 names that did not match this year; state election officials say their analysis of the data indicates that most of these are individual voters, not duplicate registrations. But Ms. Brunner said that problems with the databases could very well be why the names did not match.

“Federal government red tape, misstated technical information or glitches in databases should not be the basis for voters having to cast provisional ballots,” said Ms. Brunner, adding that she plans to require that notifications are sent to all voters whose records have discrepancies.

The Ohio Republican chairman, Robert T. Bennett, called the court ruling “a victory for the integrity of Ohio’s election.”

“Once again, Jennifer Brunner has wasted valuable taxpayer dollars only to have her partisan agenda rejected by a court of law,” Mr. Bennett said.

Daniel P. Tokaji, a law professor and voting expert at Ohio State University, said he thought the appellate decision was wrong. He said the stated purpose of the “matching” requirement in the federal law, the 2002 Help America Vote Act, was to accelerate procedures at the polls, somewhat like an E-Zpass lane at highway toll plazas. It was meant to allow voters to avoid showing identification if they had already been screened using database checks, he said.

The federal matching requirement, Mr. Tokaji said, was not meant to determine eligibility, deter voter fraud or raise added barriers for voters by forcing some to vote provisionally. “The majority judges don’t seem to grasp this point,” he said.

There is a real risk of large-scale challenges of voters on Election Day, said Richard L. Hasen, a professor at Loyola Law School in Los Angeles, but he added that any effort to use the list to purge the rolls before then could violate the federal provision that prohibits systematic voter removal purges within 90 days of a federal election.

Requiring voters to cast provisional ballots rather than regular ones is a concern because such ballots involve added verification and are often disqualified, according to voting experts.

Ms. Brunner said she was worried that requiring so many voters to cast provisional ballots would also raise tensions at the polls and worsen lines and confusion on Election Day in a year when she is expecting unprecedented turnout.

Wendy Weiser, a lawyer with the Brennan Center for Justice at the New York University law school, said many voters were flagged erroneously because the databases used to check voter registrations were prone to errors. Most nonmatches are the result of typographical errors by government officials and computer errors, she said, not voter ineligibility.