WASHINGTON - US President Barack Obama cast his ballot early in Chicago at his home state of Illinois Thursday afternoon. It was the first time that a sitting US presidential nominee voted early in person.

US President Barack Obama casts his vote early at the Martin Luther King Community Center in Chicago, Illinois, Oct 25, 2012. [Photo/Agencies]

With only 12 days left in the presidential race, Obama stepped into a polling station at around 4:20 pm local time, signed forms, showed his driver's license and made his choice at a touch-screen machine, according to White House Press Office.

"I can't tell you who I voted for, but I very much appreciate everybody here," said Obama after he cast early ballot, an effort to encourage the early voting drive of the president's reelection campaign.

"But all across the country we're seeing a lot of early voting...If something happens on Election Day, you will have already taken care of it," said Obama.

The Obama campaign has embarked on an early voting drive, as dozens of states have started early voting procedures. The president has used his campaign appearances to call on supporters to do so.  

"The President will be early voting today in Chicago. This is the first time a sitting President is early voting in person," said Jen Psaki, spokeswoman for the Obama campaign, during Thursday's press gaggle aboard Air Force One en route Richmond Virginia.

The Obama campaign said early voting has been crucial to its get-out-the-vote efforts, trying to outperform the Republican presidential ticket on the ground particularly in key swing states.

"This is a major part of our on-the-ground program and focus, and we hope that having the President do this today will send a message to people across the country, in states where early vote is an option, that this is something they should do, too," said Psaki.

"This afternoon, I'm casting my ballot in my hometown of Chicago," wrote an email by the Obama campaign sent to supporters earlier Thursday, "I'm told I'll be the first sitting president to take advantage of early voting."

The email also noted that effectively getting people out to vote, of which early voting is a huge part, would be even more critical in this year's "extremely tight race" than four years ago.

Psaki also told reporters that the Obama campaign has so far out-performed in key states compared to their own in 2008 and ahead of Republican challenger Mitt Romney right now.

A polling worker laughs after US President Barack Obama mimicked the way she checked the photo on his driver's license, as he voted early at the Martin Luther King Community Center in Chicago, Illinois, Oct 25, 2012. [Photo/Agencies]

Last week, US first lady Michelle has already cast her absentee ballot, another form of casting ballots ahead of Nov 6 other than early voting in person.

In her twitter account, Michelle Obama published a photo of her holding an absentee ballot, with the words "I just dropped my absentee ballot in the mail - I couldn't wait for Election Day!"

Following Michelle's move, Obama also announced through his official twitter account that he would vote early on Oct 25 in person.

Obama, currently on a two-day blitz campaign tour across eight states, holds advantage over Romney in early voting numbers. A Time Magazine poll released Thursday showed the incumbent led Romney two-to-one among respondents in top battleground Ohio who say they have already voted.