President Barack Obama casts his vote later in Chicago as his campaign seeks to boost early ballots in a neck-and-neck election race.
Barack Obama will be the first president to vote early, as part of a two-day campaign marathon across eight states.
His Republican challenger, Mitt Romney, is in Ohio, a swing state which could hold the key to the White House.
A new Associated Press poll suggests Mitt Romney has eked out a slight national edge over Barack Obama, by 47% to 45%.
The survey also showed that Mitt Romney had erased some of the president’s lead among women, although Barack Obama had sliced into his rival’s lead with male voters.
The president’s ballot casting on Thursday is part of the Obama campaign’s wider effort to encourage early voting, with many states holding open in-person polls this week.
First Lady Michelle Obama voted by absentee ballot on 15 October.
Because the US election is a state-by-state contest, a presidential candidate must win key battlegrounds like Ohio, Virginia and Florida, which do not reliably vote for either party. No Republican has ever won the White House without taking Ohio.
The Obama campaign recently won a court ruling to keep Ohio’s early voting open through the weekend before the election.
Mitt Romney makes three stops across the Mid-Western state on Thursday, while his running mate Paul Ryan is spending the day in Virginia.
But they have been distracted by the fall-out from a fellow Republican candidate’s remarks on Tuesday night that pregnancy from rape was part of God’s plan.
The campaign has said it disagreed with the comments by anti-abortion Indiana Senate hopeful Richard Mourdock, although it did not withdraw support from him.
“We disagree on the policy regarding exceptions for rape and incest, but still support him,” a campaign spokeswoman said.
Republicans running in tight contests elsewhere have repudiated Richard Mourdock’s remarks.
Barack Obama criticized Richard Mourdock on a US late-night talk show on Wednesday.
“I don’t know how these guys come up with these ideas… rape is rape. It is a crime,” Barack Obama said on Jay Leno show, adding that politicians had no business making decisions for women about their bodies and health choices.
On Thursday, the president makes campaign stops in Florida, Virginia and Ohio. On Monday, he will appear for the first time at a campaign event this election cycle with former President Bill Clinton.
In an interview with the Des Moines Register, Barack Obama indicated what issues would be his priority in a second term, including a budget deal to reduce the US debt, as well as immigration.
Barack Obama received a boost from former Secretary of State Colin Powell, who endorsed President Obama on Thursday.
Colin Powell, who also backed Barack Obama in 2008, cited recent improvements in the economy and the president’s guidance of the US military as reasons for his renewed support.
“I also saw the president get us out of one war, start to get us out of a second war and did not get us into any new wars,” Colin Powell said.
“I think that the actions he’s taken with respect to protecting us from terrorism have been very, very solid.”