CHICAGO (AFP/REUTERS) - President Barack Obama cast an early ballot in his adopted hometown of Chicago on Friday (Oct 7), highlighting a Democratic drive to get voters to the polls before Nov 8.

Onlookers cheered as Obama, who was a US senator from Illinois prior to becoming president, entered the early-voting facility in downtown Chicago during an unannounced visit.

A smiling Obama shook hands with the poll workers at the site and thanked them for their "outstanding work" before heading to an electronic voting machine.

"Now they can't see me, can they?" Obama asked the workers as he jokingly shielded his machine from reporters standing nearby.

The President punched in his choice and then smirked when asked who he had picked.

Democratic presidential candidate Hillary Clinton has crisscrossed key battleground states trying to get supporters to register and voter early where possible.

Obama has been campaigning for Clinton to succeed him in the Oval Office.

In 2012, he was the first president to vote early. At the time he was running for his second term in the White House.

Obama was scheduled to spend the weekend in Chicago. He made the stop at the voting facility in between appearances at two political fundraisers.

The outcome of the 2016 White House race could well be decided long before Election Day.

"We expect that this year, in the battleground states that we're focused on, at least 40 per cent of votes are going to be cast before Election Day," said Clinton campaign manager Robby Mook.

Locking down votes now, even in non-battleground states, could help the campaign tailor time and resources as the election enters the final frenetic stretch.