Obama warns Republicans about critical ads

Still, Obama faces several challenges, including the need to attract working-class voters who sided heavily with Hillary Rodham Clinton in many states. She beat him by 41 percentage points in West Virginia's primary this week, a shellacking that Obama tried to explain to reporters.

"Part of the issue with West Virginia was, we just didn't have a lot of time to get there" to campaign, he said. "I'm not well-known there. You know, some of these e-mails and rumors that we talked about have penetrated in West Virginia more deeply than they have in some other states. Debunking that stuff is relatively simple if you are on the ground talking to people. If you're not, then it's tough."

Obama said his primary losses to Clinton in Ohio, Pennsylvania and other places do not mean he cannot carry such states in November against McCain.

"I would just take a look at where the national polls are right now," he said. "We're beating McCain handily, we're doing really well among independents. There may be concerns among some voters because they don't know me that well. And I think that the longer we campaign, and the better they get to know me and my agenda, the better we'll do."