Ralph Nader has said he is considering another run for US president in 2008.
Mr Nader denied he was a "spoiler" candidate
He is blamed by many Democrats for handing George W Bush the presidency in 2000, when he cost Al Gore vital votes.
Mr Nader revealed his 2008 ambitions in US journal The Politico. His decision to run will depend on whether he can get on the ballot in all 50 states.
His announcement comes amid speculation over a possible independent run by New York's Mayor Michael Bloomberg, who left the Republicans this week.
Nearly three million Americans - more than 2% of the vote - backed Mr Nader, an anti-establishment consumer champion, when he stood as the Green Party candidate in 2000.
The election was so close, particularly in Florida and New Hampshire, that many Democrats believe Mr Gore - former President Bill Clinton's vice-president - would have won the White House had Mr Nader not taken those votes.
He stood again in 2004 but had less of an impact, winning just 400,000 votes nationwide.
Mr Nader, 73, told the New York Times that he did not see himself as a "spoiler" candidate, saying: "George Bush was the spoiler."
He said his final decision on whether or not to run would be made in the autumn and would depend on whether he could round up enough volunteers to get his name on the ballot in all US states.
He predicted the Democrats would win in 2008, provided they did not "mess it up".
Mr Nader joins a growing band of people rumoured to be considering a bid but who have not yet officially declared.
Among them are Mr Bloomberg, who insists he will serve his full term as mayor but whose departure from the Republican Party suggests he could run as an independent.
Another is Hollywood actor and former Tennessee Senator Fred Thompson, who is weighing a run for the Republican nomination and has been polling well.
The list of those thought to be possible contenders also includes Mr Gore and former House Speaker Newt Gingrich, who has said he will not make his intentions known until September.