At a campaign stop at a General Motors Corp. plant in Youngstown, Ohio, the senator from New York accused Obama of caving in to special interests.
"My opponent says that he'll take on the special interests," she said. "Well, he told people he stood up to the nuclear industry and passed a bill against them. But he actually let the nuclear industry water down his bill -- the bill never actually passed."
Clinton was referring to a 2006 bill that Obama drafted after an Illinois nuclear power plant was found to have released radiation into surrounding groundwater.
Obama's original bill would have required power plants to notify the public and government officials when any radiation was released, but subsequent versions had less stringent reporting requirements, The New York Times reported. The bill was never voted on by the full Senate.
Clinton also accused Obama of supporting "billions of dollars of breaks for the oil industry" by voting for an energy bill she opposed and said he did not support the workers of a Maytag Corp. plant that closed in his home state of Illinois.
Reacting to Clinton's charges, Obama spokesman Bill Burton said his candidate "doesn't need any lectures on special interests from the candidate who's taken more money from Washington lobbyists than any Republican running for president."
"Sen. Clinton may have said that attacks and distortions are the 'fun' and 'exciting' part of the campaign, but they're exactly what everyone else in America is tired of," Burton said.
In recent days, Clinton has challenged Obama's ability to deliver on his rhetoric.
"There's a big difference between us -- speeches versus solutions, talk versus action," she said.
"Speeches don't put food on the table. Speeches don't fill up your tank or fill your prescription or do anything about that stack of bills that keeps you up at night."
Her remarks in Ohio echo statements she made a day earlier in McAllen, Texas, when she said, "I am in the solutions business. My opponent is in the promises business."
Clinton was set later Thursday to hold events in Dayton and Columbus, Ohio. Obama was to be in his hometown of Chicago, Illinois, and had no public events scheduled.
CNN contributor and Clinton supporter James Carville said the senator must do well in the March 4 Ohio and Texas primaries if she is to stop Obama's momentum.
Carville said he thought Clinton could still win the nomination.
"You know, this thing is close. Not all the Democrats have been heard from. ... If anybody can do this, I think she can," said Carville, a major force behind President Clinton's successful 1992 campaign.
Clinton's aggressive stance may be in reaction to Obama's momentum after he won eight contests in a row -- including victories by wide margins in Tuesday's primaries in Maryland, Virginia and the District of Columbia.
In those contests, Obama also outpolled Clinton among demographic groups she had carrried earlier -- women, lower-income voters and Latinos. Clinton is banking on those groups to carry her to victory in Texas and Ohio.
The wins in the Potomac primaries gave Obama a lead over Clinton in the delegate count for the first time -- 1,253 to 1,211, according to CNN calculations.
"In my neighborhood, you know, you had to win games if you wanted to brag," said Jamal Simmons, a Democratic strategist and Obama supporter. "And I think right now, Barack Obama is ahead in delegates. He's ahead in states. He's ahead in the popular vote. He's winning.
"Sen. Clinton has got to win some of these contests, you know, to get to the finals."
Neither Clinton nor Obama is likely to pick up the 2,025 delegates needed to win the nomination outright before the primary season ends in June, mainly because Democrats divvy up each state's delegates in proportion to the candidates' share of the popular vote.
The Democratic nomination likely will be decided by the roughly 800 superdelegates, which include party officers, elected officials and activists.
Obama's camp argues the superdelegates should support the candidate with the most popular support, as indicated by a majority of pledged delegates going into the convention.
Clinton's campaign, on the other hand, says the superdelegates should support the candidate they think will be the best nominee in the general election as well as the best president.
MoveOn.org, an influential liberal activist group, on Thursday said it would launch a petition drive calling on the superdelegates not to go against the popular vote.
"The worst thing for the party and democracy is if all these new voters feel like the nomination was brokered in a backroom somewhere. The superdelegates have got to let the voters decide," MoveOn.org Executive Director Eli Pariser said in a statement.
MoveOn.org has endorsed Obama.
In a possible indication that Clinton is going to fight for every delegate, her camp on Thursday announced that daughter Chelsea Clinton would be dispatched to Hawaii to campaign before the state's primary Tuesday.
However, Chelsea Clinton may face an uphill battle there since Hawaii is Obama's native state.
Bill Clinton on Thursday was scheduled to campaign in Wisconsin, which also holds its primary Tuesday.
Recent polls in that state have shown a tight race between the two Democrats. A Strategic Vision survey conducted February 8 through Sunday finds Obama ahead of Clinton 45 percent to 41 percent, a lead outside the poll's margin of error of 3 percentage points.
Carville said Clinton could recover from a loss in Wisconsin.
"It certainly would be preferable for her to win Wisconsin, but I don't put it in the same category as I would put Texas and Ohio on March 4," he said.
CNN's Peter Hamby and Chris Welch contributed to this report.