"It's politics," the presidential candidate told a town-hall meeting on veterans affairs. "This is what we do politically, when we start getting behind in races. We start going on the attack."
Obama holds the lead in votes, pledged delegates and states won with 10 contests remaining, including the Pennsylvania primary next Tuesday.
Seeking to undercut his advantage, Clinton has seized on Obama's comments in which he told about their economic lot. Clinton also began airing an ad in Pennsylvania that shows a handful of voters saying they were insulted by what he said.
Obama has said he chose the wrong words to characterize the economic insecurity many people face.
At the town-hall meeting, an audience member said he was angry at Clinton's suggestion that Obama's comments were elitist.
"As a white person, this term, the way it's being used against you, it isn't far from 'uppity,'" the man said. "I think the Clintons are getting away with something that they must be called on. They will continue to do it until somebody states, 'Mrs. Clinton, you are really close to prejudice here.'"
Obama said he didn't believe race played a role in Clinton's strategy.
Earlier the day in Washington, D.C., Obama told a labor group that voters are justifiably angry over high gas prices, the loss of manufacturing jobs and other examples of economic insecurity, yet that is no reason to give up hope.
He returned to his signature theme of the "politics of hope" and criticized Republican rival Sen. John McCain for backing President Bush's tax cuts after opposing them.
"Just because you're mad, just because it seems like nobody is listening to ordinary Americans, that's not a reason to give up hope," Obama told the Building Trades National Legislative Conference. "You get mad and then you decide you're going to change it. If you're not angry about something you're going to sit back and let it happen to you. If you're only angry, you don't feel hopeful."
Obama pointed out that McCain opposed the Bush tax cuts, "but somewhere along the way to the Republican nomination, I guess he figured that he had to stop speaking his mind and start towing the line — because now he wants to make those tax cuts permanent."