Nader fell short of the 5 percent of the national vote needed to win federal campaign funds for his Green Party in the 2004 election. With 95 percent of the votes counted, he was drawing about 3 percent.
He said he was far from disappointed and that his populist political movement would grow.
"We're in it for the long run," he said.
At a Wednesday morning press conference, Nader was asked again whether he thinks he may have cost Al Gore the election. He replied, "I do think Al Gore cost me the election."
Defiant to the end, the consumer advocate turned presidential candidate finished his bid for the White House the way he started, pushing for every vote and brushing off complaints by Democrats that he was a spoiler, drawing votes away from Al Gore.
"You can't spoil a system spoiled to the core," he declared Tuesday night to a standing-room crowd in the ballroom of the National Press Club.
Nader addressed the gathering after quelling their chants of "Go, Ralph, Go!" Standing beside him were his two sisters, a niece and his mother, Rose Nader, who blew the group a kiss.
Nader kept up his appeals for supporters to "vote their conscience," insisting the vice president had to earn his votes like everyone else. In states with close races between Gore and Republican George W. Bush, Nader appeared to be a significant factor.
Exit polls in states including Colorado, Florida, Nevada, New Hampshire, Oregon, Washington state and Wisconsin suggested that at least half the Nader voters would have voted for Gore if it had been a two-way race. In many of those states, it was enough to throw the state to Bush. But nearly one in three said they simply would not have voted at all.
Voters were interviewed as they left the polls by Voter News Service, a consortium of The Associated Press and the television networks.
Fearing Nader's popularity would cost Gore the election, Democrats tried convincing Nader supporters that "a vote for Nader is a vote for Bush."
John Gehan of St. Paul, Minn., didn't buy that argument, and he challenged anyone to call his vote for Nader a mistake.
"Those sort of attacks on him and me solidified my vote for Nader," said Gehan, 51. "If Gore loses, it's because Gore is a lousy candidate, not because I voted for Nader."
He said Nader presented the only choice for progressive Democrats.
Nader's perceived role as a potential spoiler also put him under pressure from longtime friends and allies once closely aligned with his progressive views.
But he remained defiant and unapologetic, urging people to support a "viable third party" hat would serve as a watchdog for 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," Nader said earlier Tuesday during a news conference in front of the Liberty Bell in Philadelphia.
Nader's long-shot campaign emphasized his criticism of big business and the two-party political system. He has long insisted that no major differences exist between the Democrats and the Republicans or their presidential candidates.
"The two parties have morphed into a corporate party representing the same business interests at the same dinners, at the same hotels, day after day after day," he said.
Nader felt his candidacy was badly hurt by his exclusion from the presidential debates. Sponsors required 15 percent support in national polls. Yet, he campaigned aggressively, holding rock concert-like rallies that attracted thousands of people paying an average of $10 apiece in Boston, Chicago, New York, Seattle, Washington and other cities.
Nader is best known as the consumer advocate who in the 1960s took on the automobile industry's safety standards. Since then, he has pushed for automatic seat belts and other safety devices in cars and for passage of federal legislation to improve the air, food, water and the environment.
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