Ralph Nader says he will run again as an independent for the US presidency.
The anti-establishment consumer advocate made the announcement in a televised interview on Sunday.
Mr Nader was accused by many Democrats of handing the presidency to George W Bush in the November 2000 elections. He ran again unsuccessfully in 2004.
Senators Barack Obama and Hillary Clinton are vying for the Democratic ticket. Senator John McCain is almost certain to run for the Republicans.
Nearly three million Americans - more than 2% of the vote - backed Mr Nader when he stood as the Green Party candidate in the 2000 presidential election.
That election was so close that a small proportion of those votes - particularly in the key state of Florida - could have helped to put Al Gore in the White House.
"I'm running for president," Mr Nader said as he announced the move on NBC's Meet the Press.
He said most Americans were disenchanted with the Democratic and Republican parties - who were not discussing the urgent issues facing American voters
People, he said, felt "locked out, shut out, marginalised and disrespected".
He called Washington DC "corporate-occupied territory" that turns the government against the interest of its own people.
Mr Nader denied he was seeking to be a spoiler candidate - and accused the main parties of "political bigotry".
Referring to the three main contenders in the race so far, he questioned: "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?"
"We have to shift the power from the few to the many."
Speaking to the BBC, Mr Nader said the two major parties "don't stand for the people".
"They stand for big business. That's overwhelmingly documented. And they have not cracked down on the corporate crime-wave that the Wall Street Journal and The Washington Post and the networks have documented day after day, the Enron scandal all the way to Wall Street."
HAVE YOUR SAY
I'm all about a viable third party candidate but the key word here is viable. Nader is not the one, it almost like the Republicans have him on retainer.
Eric H., Maine, US
Democratic candidates Barack Obama and Hillary Rodham Clinton were quick to criticise Mr Nader.
"He thought that there was no difference between Al Gore and George Bush and eight years later I think people realise that Ralph did not know what he was talking about," Mr Obama was quoted by the Associated Press as saying.
"I remember when he did this before, it didn't turn out too well, for anyone, especially our country," Mrs Clinton said.
"I hope it's kind of a just a passing fancy that people won't take too seriously."
Republican hopeful Mike Huckabee was much more welcoming:
"I think it always would probably pull votes away from the Democrats, not the Republicans, so naturally Republicans would welcome his entry into the race and hope that maybe a few more will join in," he told CNN.
Mr Nader, 73, was born in Connecticut in 1934 and was educated at Princeton and Harvard universities.
He has spent most of his life fighting for consumers and workers against corporations.
In the 1960s his work on car safety led directly to seat belts and shatter-resistant glass being fitted in every American car.
From the 1970s he built a reputation for dealing with issues including workers' rights, public safety, the environment and the influence of corporations.
He founded a number of groups including Public Citizen, which in recent years has been active in organising protests against the World Trade Organization and World Bank/IMF.