WASHINGTON - As President Obama and GOP challenger Mitt Romney spent election eve rushing from one swing state to another, their campaigns were already preparing for a complicated -- and possibly long -- battle over Tuesday's vote.

Even before Tuesday's voting began, the two sides were skirmishing over how the balloting was being administered.

In Ohio, a new dispute has broken out over the validity of provisional ballots. Usually, such special ballots -- cast by voters but set aside for examination later -- are required when something about the voter's eligibility is in doubt. For example, the voter might lack proper ID or be in the wrong precinct, or the person might have requested an absentee ballot but then showed up to vote in person at a polling place.

When examined in more detail later, provisional ballots are either discarded or, if the voter's eligibility is established, counted. The fight over those ballots has now increased the possibility that -- if Tuesday's election comes down to the Buckeye State, it won't end on Tuesday night at all.

Instead, it might be weeks before Ohio has a final result. Voting rights advocates contend that a new directive issued Friday by Ohio Secretary of State Jon Husted improperly places the burden on voters -- rather than poll workers -- for accurately recording the form of identification on provisional ballots.

Husted ordered the state's 88 county elections boards to reject provisional ballots when the ID portion is incomplete. This appears to be in conflict with a consent decree reached last month between the state and voting rights groups that said provisional ballots with incomplete identification information should be counted.

A group of unions and voting rights groups went to federal court Thursday asking that the state be made to reaffirm that commitment. A day later, Husted released his directive. A ruling, from a U.S. district court judge will not come in time for Election Day. But he is expected to decide the case by Nov. 17, when Ohio counts provisional ballots.

Election boards have 10 days after the election to evaluate the eligibility of provisional ballots and decide whether to count them.

In its final projection before Tuesday's vote, the Ohio Poll sponsored by the University of Cincinnati found the presidential race in Ohio too close to call, with Obama receiving support from 50 percent of probable voters and Romney getting 48.5 percent -- within the poll's margin of error of plus or minus 3.3 percent.

That represented no significant change from last week's Ohio Poll, which gave Obama a 2-point edge over Romney. However, the latest NBC News/Wall Street Journal/Marist poll released Monday gives the president a 6-point lead in Ohio, 51 percent to 45 percent.

On Tuesday night, parsing the early returns from Ohio could be confusing. In the first minutes after polls close, the state is likely to tally up the returns from early voting. These are expected to break heavily for Obama. After that, Romney should creep closer, since he is expected to do better among those who vote on Election Day.

By the time all those votes are counted, the winner still may not be clear. If the number of provisional ballots cast is greater than the number of votes that separate the two candidates, then there could be a long, heated battle over which provisional ballots to count.

Florida's voting problems

The last days of early voting in Florida this past weekend were marked by long lines, a bomb scare, a flurry of lawsuits and general confusion. In Orange County, a judge extended voting hours on Sunday after a suspicious package -- a cooler -- shut down an early voting site at the Winter Park Public Library on Saturday.

Democrats filed suit late Saturday to extend the hours. Republicans did not challenge the judge's decision.

Long lines -- created in part by ballots as long as 12 pages (six pages, both sides) -- were a big part of the problem elsewhere in the state.