The law curtailing early voting and preventing people from registering to vote and casting ballots on the same day illegally discriminates against black voters, Judge Michael Watson of Federal District Court in Cincinnati, who was appointed by President George W. Bush, ruled on Tuesday.
The decision orders the crucial swing state to change its rules before the fall election.
Early voting in Ohio grew out of the 2004 election, when people stood in line for hours to cast ballots, especially in heavily minority and Democratic urban areas. Thousands of people went home without voting, says The New York Times.
George W. Bush won Ohio by 2.1 percentage points. If he had lost the state, John Kerry would have become president, the Times says.