WASHINGTON — Ralph Nader said today he will run for president as a third-party candidate, criticizing the top White House contenders as too close to big business and pledging to repeat a bid that will “shift the power from the few to the many.”

Mr. Nader, 73, said most people are disenchanted with the Democratic and Republican parties due to a prolonged Iraq war and a shaky economy. The consumer advocate also blamed tax and other corporate-friendly policies under the Bush administration that he said have left many lower- and middle-class people in debt.

“You take that framework of people feeling locked out, shut out, marginalized, disrespected,” he said. “You go from Iraq, to Palestine/Israel, from Enron to Wall Street, from Katrina to the bungling of the Bush administration, to the complicity of the Democrats in not stopping him on the war, stopping him on the tax cuts.”

“In that context, I have decided to run for president,” Mr. Nader told NBC’s “Meet the Press.”

Mr. Nader also criticized a Senators McCain, Obama, and Clinton for failing to support full Medicare for all or cracking down on Pentagon waste and a “bloated military budget.” He blamed that on corporate lobbyists and special interests, which he said dominate Washington, D.C., and pledged in his third-party campaign to accept donations only from individuals.

“The issue is do they have the moral courage, do they have the fortitude to stand up against the corporate powers and get things done for the American people,” Mr. Nader said. “We need to shift the power from the few to the many.”

Mr. Nader also ran as a third-party candidate in 2000 and 2004, and many Democrats still accuse him of costing Al Gore the 2000 election.

Mr. Obama spoke dismissively of Mr. Nader at a news conference in Ohio, referring to him as a perennial presidential campaigner.

“He thought that there was no difference between Al Gore and George Bush and eight years later I think people realize that Ralph did not know what he was talking about,” Mr. Obama said.

Mrs. Clinton called Mr. Nader’s announcement a “passing fancy” and said she hoped his candidacy wouldn’t hurt the Democratic nominee.

“Obviously, it’s not helpful to whomever our Democratic nominee is. But it’s a free country,” she told reporters as she flew to Rhode Island for campaign events.

A Republican presidential candidate, Michael Huckabee, speaking shortly before Mr. Nader’s announcement, said Mr. Nader’s past runs have shown that he usually pulls votes from the Democrat. “So naturally, Republicans would welcome his entry into the race,” the former Arkansas governor said on CNN.

Mr. Nader vociferously disputes the spoiler claim, saying only Democrats are to blame for losing the race to George W. Bush. He said today there could be no chance of him tipping the election to Republicans because the electorate will not vote for a “pro-war John McCain.”

“If the Democrats can’t landslide the Republicans this year, they ought to just wrap up, close down, emerge in a different form,” Mr. Nader said.