Ralph Nader: Fifth consecutive campaign for the presidency
Ralph Nader is the man accused by many Democrats of handing the US presidency to George W Bush in November 2000.
Nearly three million Americans - more than 2% of the vote - backed the anti-establishment consumer advocate when he stood as the Green Party's candidate.
The election was so close that a small proportion of those votes - particularly in the key state of Florida - would have put Al Gore in the White House.
He ran again for the presidency unsuccessfully in 2004. On that occasion, he stood as independent candidate endorsed by the Reform Party and won only a tiny fraction of the popular vote - 0.38%.
On Sunday, the 73-year-old revealed he would run for a fifth consecutive time, even though yet again he will be seen as a candidate with virtually no chance of winning.
Voters 'shut out'
Launching his 2008 campaign, Mr Nader said most Americans were disenchanted with the Democratic and Republican parties, who were not discussing the urgent issues facing American voters.
"You take that framework of people feeling locked out, shut out, marginalised and disrespected," he told NBC's Meet The Press.
"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."
Mr Nader said the country needed an infusion of freedom, democracy, choice and dissent, and refused to let the two main parties "own all the voters and turn over the government to big business".
"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," he added. "We have to shift the power from the few to the many."
The consumer advocate denied he was seeking to be a spoiler candidate in a hard-fought campaign, saying that if either Democratic candidate could not win by a "landslide" this year "they should just close down".
In 2004, a slick website called Don't Vote Ralph was set up to encourage left-wing voters not to support Mr Nader, arguing he might take valuable votes away from the then Democratic candidate, John Kerry.
But he refused to back down and insisted he would "take more votes from Bush". Not for nothing has one of his friends described him as one of the most stubborn men in the US.
New left-wing politics
Mr Nader was born in Winsted, Connecticut, in February 1934, the son of Lebanese immigrants.
After studying International Affairs at Princeton University in 1955 and graduating from Harvard Law School three years later, he moved to Washington, where he continues to live in a modest apartment.
Mr Nader has spent most of his life fighting for consumers and workers
His website says he "was taught by his parents that social activism was a civic duty".
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 Organisation and World Bank/IMF.