About 30 percent of the swing state's total vote - or roughly 1.7 million ballots - came in before Election Day in 2008. Crucial to Obama's win that year was early voting in Ohio, North Carolina, and Florida.
Obama won Ohio four years ago, but Republican rival Mitt Romney is making a strong play for it this year. No GOP candidate has won the White House without Ohio in his column.
Obama's campaign and Ohio Democrats had sued state officials over changes in state law that took away the three days of voting for most people but made exceptions for military personnel and Ohioans living overseas.
Their lawsuit cited a recent study saying nearly 105,000 people voted in the three days before the election in 2008, and they argued that everyone should have the chance to vote on those days.
They also said that eliminating the opportunity for most Ohio residents to vote in person on those days, while giving military or overseas voters the chance to do so, leads to unequal treatment.
Attorneys for the state said many laws already grant military personnel special voting accommodations, such as requirements for states to send absentee ballots to them 45 days before the election. And they argued that local boards also need those final days to prepare for the election.
On Oct. 5, a federal appeals court reinstated voting on the weekend and Monday before the election and returned discretion to set hours on those days to local boards of elections.
Ohio Secretary of State Jon Husted appealed that ruling to the Supreme Court last week.
He said that it opened up the chance for Ohio's 88 county boards of elections to set different rules, while simultaneously ordering that all voters be treated the same.
But the high court in a one-sentence ruling allowed the lower court's ruling to stand.
Before the changes to the Ohio law, local boards of elections set their own early voting hours. And those hours varied among the state's counties.
Husted had been accepting boards' recommendations for hours on the disputed days in the event his appeal wasn't successful.
About an hour after the high court's decision, Husted ordered uniform hours across the state. The hours are from 8 a.m. to 2 p.m. on Saturday, Nov. 3; from 1 p.m. to 5 p.m. on Sunday, Nov. 4; and from 8 a.m. to 2 p.m. on Monday, Nov. 5.
Husted said that despite the high court's ruling, he believes the Ohio legislature, not the federal courts, should set the voting rules.
"However, the time has come to set aside the issue for this election," he said in a statement.
Husted said the new statewide hours on the three days would give Ohioans the same opportunities to vote regardless of their home county.