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 and disrespected," he said. "You go from Iraq, to Palestine to Israel, from Enron to Wall Street, from Katrina to the bumbling 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," Nader told NBC's "Meet the Press."
Nader also criticized Republican candidate John McCain and Democrats Barack Obama and Hillary Rodham 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 to corporate powers and get things done for the American people," Nader said. "We have to shift the power from the few to the many."
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.
Obama, responding Saturday to Nader's earlier criticisms that he lacked "substance," praised Nader as a "heroic figure."
"In many ways he is a heroic figure and I don't mean to diminish him. But I do think there is a sense now that if somebody is not hewing to the Ralph Nader agenda, then you must be lacking in some way," Obama said.
Clinton called 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.
Republican presidential candidate Mike Huckabee, speaking shortly before Nader's announcement, said 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.
Nader vociferously disputes the spoiler claim, saying only Democrats are to blame for losing the race to George W. Bush. He said Sunday 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," Nader said.
His campaign Web site can be found at www.votenader.org.