Early voting is underway in the state of Ohio - exactly five weeks before the US presidential election on 4 November.
Ohio courts ruled on Monday that new voters could register and cast an absentee ballot on the same day.
Republicans, who argue same-day voting opens the door to voter fraud, opposed the measure; Democrats backed it.
Both John McCain and Barack Obama have been campaigning hard in Ohio, which is seen as a key swing state.
Senator McCain, accompanied by running mate Sarah Palin, used a rally in Columbus, Ohio, on Monday to attack his Democratic rival over his economic policies. \\
Mrs Palin, Republican governor of Alaska, is preparing for a vice-presidential debate scheduled for Thursday night, in which she and Senator Joe Biden will face each other for the first time.
The BBC reporte she is under increasing pressure after a series of TV interviews which critics say have exposed her lack of readiness for the role.
Both Senator Obama's campaign and the Republican National Committee have urged supporters to make use of Ohio's 35-day early voting period to cast their absentee ballots.
Ballots can be sent by post to voters' county election boards or delivered in person to designated places.
Ohio was an important swing state in 2004, when it went to President George Bush by a narrow margin, helping him to re-election.
A number of states, including the battleground states of Iowa, Virginia, Nevada, New Mexico and North Carolina, allow early and absentee voting.