Clinton, who trails Illinois Sen Obama in delegates to this summer's national convention that will pick the Democratic candidate for the November election, needs wins in both Texas and Ohio to keep her campaign afloat.
"I'm working as hard as I can," the New York senator told reporters in Rhode Island, which also votes on March 4. "I have good campaigns in Texas and Ohio and I feel really positive about what's going to happen on March 4."
The former first lady, who would be the first woman US president, toughened her attacks on Obama and some leaflets he circulated in Ohio criticizing her health care plan and past support for the North American Free Trade Agreement.
"Nobody believes Senator Obama's plan is universal because it's not. Mine is," she said in Rhode Island, which also votes on March 4. "So raise legitimate questions but don't engage in, you know, this kind of false and misleading advertising."
"There's a big difference between what is said in that campaign and what is done in that campaign," she said.
Obama, who would be the first black president, has said Clinton's anger was just a frustrated campaign tactic since the leaflets had been in circulation for several weeks and she had not complained before.
But Clinton said she thought the Obama campaign had withdrawn them after her team pointed out errors so she was surprised when a woman handed her one Saturday in Ohio.
"I thought they'd stopped," Clinton said. "They had been discredited and we'd called their hand and I thought they'd stopped, or at least that it would have been revised."
Obama fired back in Lorain, Ohio, criticizing Clinton for changing her position on NAFTA pushed through by her husband, former President Bill Clinton.
"Senator Clinton's premise in her candidacy throughout this campaign has been 35 years of experience, including eight years in the White House, right? She has essentially presented herself as co-president during the Clinton years," he said.
"So the notion that you can selectively pick what you take credit for and then run away from what isn't politically convenient, that doesn't make sense," he said.
A FAMILIAR FACE
As the two U.S. senators went after each in the race to be the Democratic nominee in November's election, a familiar face joined the presidential race. Consumer advocate Ralph Nader, blamed by many Democrats for their 2000 White House loss, said he would run again as an independent.
Nader ran as an independent in 2004 and as the Green Party nominee in 2000 when he won enough votes in Florida to play a part in Democratic presidential nominee Al Gore's loss of that state and the White House.
Nader called Washington "corporate occupied territory" that turns the government against the interests of the people. "In that context, I have decided to run for president," he said.
Democrats dismissed his announcement.
Clinton called Nader's decision "a passing fancy" and said he had "prevented Al Gore from being the greatest president we could have had and I think that's really unfortunate."
Obama, who has won 10 straight Democratic contests, hopes to knock off Clinton in either Ohio or Texas, where she once held big leads. The two face off in their last scheduled debate on Tuesday in Ohio.
In the Republican race, reaction to a New York Times article last week continued to reverberate. The Times hinted at the possibility that presidential front-runner John McCain was having a romantic affair in 1999 with a female lobbyist 31 years his junior.
McCain, the Arizona senator who has all but clinched the Republican nomination, has said the story was untrue.
Conservatives railed at the Times for trying to smear McCain with a story based on unidentified sources. On Sunday, they were joined by the Times' own public editor.
"If a newspaper is going to suggest an improper sexual affair ... it owes readers more proof than The Times was able to provide," wrote Clark Hoyt, who writes a weekly critique.
(Writing by David Wiessler; Additional reporting by Jeff Mason, Donna Smith; Editing by Chris Wilson)