“We’re expecting 200,000 or more provisional ballots — that’s more than New York or California — and that means that an election is contestable here with a margin in the low tens of thousands of votes,” Mr. Tokaji said.

Voting rights groups and the Service Employees International Union have complained that Ohio’s Republican secretary of state, Jon Husted, has set voter identification rules for provisional ballots that are overly strict and violate the law. Mr. Husted said that his directive is consistent with the law and past practice in Ohio. A federal judge on Monday set a hearing for Wednesday on the dispute so that it can be resolved before counting of provisional ballots begins.

Ohio does not begin to tally provisional ballots and ballots from overseas and military voters until Nov. 17. During this 10-day period, voters can provide documentation to establish their eligibility, but election officials cannot open or count absentee or provisional ballots. In 2008, 80 percent of those votes ended up being accepted as legitimate, and they tended to slightly favor the Democratic candidate, Mr. Tokaji said.

Ohio counties then have an additional 10 days to conduct an official canvass of the vote. If at the end of that period the two candidates are within 0.25 percent of the overall vote total, an automatic recount is triggered. State officials expect about 5.7 million votes to be cast in the presidential election, meaning that if the margin is 14,250 votes or fewer, a statewide recount is required. Under state law, that cannot begin until Dec. 2 and must end by Dec. 11, six days before the state must qualify its representatives to the Electoral College. While much of the focus is on Ohio, the result could also come down to several other highly contested states, each with its own set of rules for counting or recounting votes.