"I Am Not Usain Bolt" - Says President Barack Obama

US President Barack Obama has warned supporters that he is no Usain Bolt, and that his re-election bid would be a fight all the way to the finish line, not an easy jog to victory.

Obama ushered well-heeled supporters into his Georgian-style mansion in Chicago's Hyde Park neighbourhood for a rare glimpse of the home he left behind when he won the White House in 2008.

He offered a pep talk to the crowd of around 100 of Chicago's elite, who had paid $40,000 a head for the exclusive invitation designed to swell his campaign coffers ahead of the presidential election on November 6.

The president predicted a tight struggle with Republican Mitt Romney as he asks voters for a second term.

"This is not going to be a race like Usain Bolt where we're like 40 yards ahead and we can just kind of start jogging 10 feet before the finish line," Obama said referencing the Jamaican legend, who has won the 100 meters and 200 meters sprint double at the last two Olympics.

"We're going to have to run through the tape. But we're really well positioned to not just win but to keep America moving forward."

"This year, you have a stark choice, as stark as we've seen in a generation, at least," Obama said on a small, hedge-fringed stage set up on his lush front lawn.

"On behalf of Michelle and myself, I want to thank you but I also want to warn you: this is going to be a close election. We've got 86 days left. Now is not the time to get weary. Now is the time to double down."

The curtains of the president's mansion were drawn, so it was not possible to see inside the house, framed by an ornate porch with white columns.