In an interview with The Associated Press, Obama said Romney lacks serious ideas, refuses to "own up" to the responsibilities of what it takes to be president, and deals in factually dishonest arguments that could soon haunt him in face-to-face debates.

Obama also offered a glimpse of how he would govern in a second term of divided government, insisting rosily that the forces of the election would help break Washington's stalemate. He said he would be willing to make a range of compromises with Republicans, confident there are some who would rather make deals than remain part of "one of the least productive Congresses in American history."

With the remarks, Obama set up a contrast between Romney, whom he cast as an extremist pushing staunchly conservative policies, and himself, by saying he would work across party lines. It was a seeming play for the independent voters who decide close elections and tell pollsters they want to see the often-gridlocked politicians in Washington solve the nation's problems.

Mainly, Obama was intent on countering Romney even before his challenger got to the Republican National Convention, which starts Monday in Tampa, Fla. (Read More: GOP Convention Delayed by Tropical Storm Isaac) In doing so, the president depicted his opponent as having accumulated ideas far outside the mainstream with no room to turn back.

"I can't speak to Gov. Romney's motivations," Obama said. "What I can say is that he has signed up for positions, extreme positions, that are very consistent with positions that a number of House Republicans have taken. And whether he actually believes in those or not, I have no doubt that he would carry forward some of the things that he's talked about."

Obama spoke to the AP on Thursday before heading off to a long weekend with his family at Camp David, the secluded presidential retreat in the Maryland mountains.

The president was at ease but doggedly on script, steering even personal-themed questions about Romney and running mate Paul Ryan into answers about starkly different visions for helping the middle class. (Read More: Paul Ryan)