5. Late-arriving votes: If the race is still close once all the precincts report unofficial results, probably sometime Wednesday, it could be weeks before it is known whether Obama or Romney won Ohio.

In 2008, Obama widened is victory over John McCain by 56,912 votes between the available results the Wednesday afternoon after the election and the final tally weeks later. Nearly 500,000 votes were added to tht totals.

This was in large part due to late-arriving absentee ballots (if mailed in time, they have 10 days to get to the boards of elections) and provisional ballots cast for various reasons. This year, the county boards of elections must certify their results by Nov. 27.

2008 vote: McCain vs. Obama Total Votes Obama Lead Obama Pct
Wednesday after election 5,217,434 205,312 51.16%
Votes added later 490,852 56,912 55.12%
Final official vote 5,708,286 262,224 51.50%