Barack Obama has become the first President to cast his ballot before Election Day.

Obama voted in his hometown of Chicago and joked with electoral staff while having his ID checked.

"I'm just glad I renewed my driver's license," he said after a poll worker asked to see his photo ID and studied it carefully.

The Obama campaign has taken the strategic decision to vote early hoping it will encourage others to do the same. There are fears many who voted for the former Illinois senator previously will not bother this time around.

His Republican rival Mitt Romney has been on a charm offensive across Ohio, which is seen as key to unlocking the White House.

Romney will be reminded constantly that no Republican has become President without winning Ohio.

At a rally in Cincinnati he said Obama's administration has had a detrimental effect on people's pockets.

"The path we are on has the average income of American families down by $4300 per year over the president's term. The path we are on doesn't have new answers. The president has the same old answers as in the past, he wants another stimulus, he wants to hire more government workers and he wants to raise taxes."

Polls show Obama and Romney neck and neck

A Reuters/Ipsos online tracking poll gave Romney a 1-point edge on Wednesday, 47 per cent to Obama's 46 per cent.