Nader denies he 'locked' Gore out of White House
In Florida's knife-edge ballot, Mr Nader polled more than 95,000 votes, or 2% of the vote.
The Democrats have accused the Green Party of syphoning off votes from their campaign, arguing that "a vote for Nader is a vote for Bush".
Rejecting these suggestions, Mr Nader ended his bid for the White House by saying the Green Party had reached a take-off stage despite performing poorly in the elections.
His party fell short of the 5% of the national vote needed to qualify for federal campaign funds in the 2004 elections.
"The important thing here is that we have reached a take-off stage in the Green Party and that this is the last time that the two parties ...will have the monopoly power," he said.
Mr Nader said he was not disappointed that his party had failed to garner enough votes to secure state funding for its future elections, saying "we're in it for the long run."
Spoiled system
Exit polls in Colorado, Nevada, New Hampshire, Oregon, Washington State and Wisconsin suggested that at least half of Mr Nader's voters would have voted for Mr Gore had it been a two-way race.
Mr Nader urged people to support a third party that would serve as a watchdog of Republicans and Democrats long after election day.
"I did not run for president to help elect one or the other of the two major candidates", the Green Party leader told a news conference on Tuesday.
"You can't spoil a system spoiled to the core," said Mr Nader, a legendary consumer advocate.
Throughout his campaign, Mr Nader appealed to his supporters to "vote their conscience," even in states closely contested by Mr Gore and Mr Bush.