Nader said most people are disenchanted with the Democratic and Republican parties due to a prolonged Iraq war and a shaky economy. He 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."
Nader, who turns 74 later this week, announced his candidacy on NBC's "Meet the Press."
In a later interview with The Associated Press, he rejected the notion of himself as a spoiler candidate, saying the electorate will not vote for a "pro-war John McCain." He also predicted his campaign would do better than in 2004, when he won just 0.3 percent of the vote as an independent.
"This time we're ready for them," said Nader of the Democratic Party lawsuits that kept him off the ballot in some states.
Democratic candidates Barack Obama and Hillary Rodham Clinton quickly sought to portray Nader's announcement as having little impact.
"Obviously, it's not helpful to whomever our Democratic nominee is. But it's a free country," said Clinton, who called Nader's announcement a "passing fancy."
Obama dismissed Nader 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," Obama added.
Republican Mike Huckabee welcomed Nader into the race.
"I think it always would probably pull votes away from the Democrats, not the Republicans," the former Arkansas governor said on CNN.
Nader said Obama's and Clinton's lukewarm response was not surprising given that both political parties typically treat third-party candidates as "second-class citizens." Nader said he will decide in the coming days whether to run as an independent, Green Party candidate or in some other third party.
Pointing a finger at Republicans, he described McCain as a candidate for "perpetual war" and said he welcomed the support of Republican conservatives "who don't like the war in Iraq, who don't like taxpayer dollars wasted, and who don't like the Patriot Act and who treasure their rights of privacy."
"If the Democrats can't landslide the Republicans this year, they ought to just wrap up," Nader added.
