WASHINGTON — The Supreme Court handed an advantage to the Democrats and President Barack Obama on Tuesday in ruling that the key battleground state of Ohio cannot limit early voting in the final days before the election.

The decision comes two weeks after a judge ordered state officials not to enforce a controversial voter ID law in another key battleground, Pennsylvania.

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Critics say the voter ID laws implemented in 30 US states are designed to keep away the poor, non-whites and the elderly — all typical Democratic supporters.

Early voting also tends to favor Democrats by making it easier for people with restricted access to transportation or strict work schedules — who also tend to be poor, minorities, and older voters — to cast ballots.

Ohio implemented early voting after the 2004 election fiasco when huge lines formed at polling stations — and countless voters gave up and went home.

But the Republican-led Ohio legislature changed the rules last year so that only members of the military could cast in-person ballots after 6:00 pm on the Friday before the election.

Ohio Secretary of State Jon Husted also ordered local polling stations to only operate during normal business hours, thus preventing early voting in the evenings and on weekends. He argued the rules were necessary in order to help poll workers prepare for the November 6 election.

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The Obama campaign challenged the rules in court, arguing that allowing members of the military but not the general public to vote on the weekend before election day violated constitutional guarantees to equal treatment.

A federal appeals court found in Obama’s favor and the Supreme Court denied Husted’s appeal in a single sentence: “The application for stay presented to Justice Kagan and by her referred to the Court is denied.”

Early voting has been a boon for Ohio Democrats, whose mobilization efforts included bussing supporters to the polls after church on the Sunday before the election.

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Some 105,000 Ohioans cast their ballots in person during the final three days before the 2008 election, in which 30 percent of votes were cast early. Obama carried the state by just over 200,000 votes.

The Obama campaign hailed the Supreme Court’s decision as a “victory for voting rights.”

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“This action from the highest court in the land marks the end of the road in our fight to ensure open voting this year for all Ohioans, including military, veterans, and overseas voters,” General Counsel Bob Bauer said in a statement.

The ruling does not require local officials to open polling stations — it simply allows them to do so contrary to Husted’s previous orders.

Yet it could still have a significant impact on the number of ballots cast.

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“The court’s decision could help determine who will win the White House,” Richard Hasen, a professor at the University of California Irvine who specializes in election law, wrote in Slate on Monday ahead of the ruling.

Husted did not immediately respond to the court’s ruling, but his office did post new voting hours which will be uniformly applied in all counties that allow for in-person voting in the final days before the election.

He has called the district court’s ruling “federal overreach” into state election law and was joined by election officials from 15 states in urging the Supreme Court to overturn the ruling.

“If allowed to stand, the precedent set by this decision in the state of Ohio will have far reaching consequences for all 50, whether they are ‘red,’ ‘blue,’ or ‘swing’ states,” Husted said in a statement Friday.