President Barack Obama chided himself on Sunday for his weak debate performance against Republican Mitt Romney and reminded supporters at a California fundraiser that even his successful 2008 campaign had "bumps" in the road.

Speaking to a crowd of some 6,000 at a concert in Los Angeles, Obama complimented the slew of performers - including Jon Bon Jovi and Jennifer Hudson - who had performed earlier for the audience.

"They just perform flawlessly night after night. I can't always say the same," Obama said to laughter and then applause.

Romney's strong performance at the debate on Wednesday has helped close a gap in the polls with the president, who has been leading nationally and in several important swing states.

Obama, whose campaign has said it would make adjustments as a result of the debate, noted his White House win four years ago had its own share of hurdles, including mistakes he made.

"Back in 2008 - everybody always remembers the victory, but they don't always remember the bumps in the road," he said.

"Things always look good in retrospect. But in the middle of it ... we made all kinds of mistakes. We goofed up. I goofed up. But the American people carried us forward," he said.

At another, more intimate fundraiser later in the evening, Obama pledged to work harder than ever to gain re-election, in what could have been an effort to reassure his supporters after the lackluster debate performance.

"I'm a big believe in closing the deal," he said to applause. "You will see me working as hard as I've ever worked for the next ... 30 days," he said.

The president noted that the encounter with Romney had happened on his 20th wedding anniversary, a fact that he offered up as another potential reason for his poor performance.

"The actual anniversary date was not that romantic," he said to laughter from the crowd, which included actor George Clooney.

"There was some speculation as to whether this had an impact on my performance."

Obama is holding a series of fundraisers in California in an effort to boost his campaign coffers for the final stretch of the race against Romney. On Saturday, his campaign announced that it, along with Democratic Party allies, brought in a record $181 million in September.