PROVIDENCE, Rhode Island - Democratic presidential hopefuls Hillary Clinton and Barack Obama sharpened their attacks on each other on Sunday, trading barbs over health care, trade and experience as they head for key showdowns in Texas and Ohio on March 4.

As Obama tried to nail down the Democratic nomination by winning those two states and Clinton battled to say alive, 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.

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 states to keep her campaign afloat.

"I could just stand up here and say 'Let's just get everybody together, let's get unified.' The sky will open, the light will come down, celestial choirs will be singing and everyone will know we should do the right thing and the world will be perfect," she said at a rally in Providence, Rhode Island.

Obama fired back in Lorain, Ohio, criticizing the New York senator for changing her position on the North American Free Trade Agreement pushed through by her husband, former President Bill Clinton.

"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."

With the economy a key issue in the U.S. presidential race, Obama has turned trade into a centerpiece of his campaign in Ohio, where trade agreements are particularly unpopular as domestic manufacturing jobs disappear.

The former first lady, who would be the first woman U.S. president, resumed her attacks on Obama over some campaign leaflets he circulated in Ohio criticizing her health care plan and past support for NAFTA.

"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."

Nader dismissed
Democrats dismissed the announcement of Nader's candidacy.

Nader, who turns 74 this week, ran as an independent in 2004. He was the Green Party nominee in 2000 when he won about 2.7 percent of the votes nationwide, but enough 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.

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."

Virginia's Democratic Gov. Tim Kaine told "Fox News Sunday: "I wouldn't see it having any effect on the race."

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.