Ron Paul gained a big internet following during the Republican primaries
Texas Congressman Ron Paul has urged voters to reject Democrat Barack Obama and Republican John McCain and vote for a third-party US candidate instead.
Dr Paul said a former senior McCain adviser had asked him this week to endorse Mr McCain - but he had refused.
He is backing independent Ralph Nader, Libertarian Bob Barr and Green Party and Constitution Party candidates.
Dr Paul, who dropped his own bid for the Republican nomination earlier this year, has a strong internet following.
He told a news conference in Washington he had been contacted by ex-McCain adviser Phil Gramm, who was dropped from the campaign after he said the US was a "nation of whiners" over the economy.
Dr Paul said Mr Gramm told him: "You need to endorse McCain." He refused.
"The idea was that he would do less harm than the other candidate," Dr Paul added, quoted by the Associated Press news agency.
Dr Paul said the majority of Americans were unhappy with the choice on offer from the mainstream parties and urged the third-party candidates to bring their supporters together to vote against the "establishment candidates".
Mr Nader, Green Party candidate Cynthia McKinney, a former Democratic congresswoman for Georgia, and Constitution Party hopeful Chuck Baldwin were among those to attend the news conference.
However, Mr Barr turned down the invitation, saying at his own news conference later that he had done so because Dr Paul failed to endorse one specific candidate.
Ahead of Dr Paul's event, Mr Nader was quoted by AP as saying it would "raise the eyebrows" of commentators sceptical about their chances in November's presidential election.
"This is the beginning of the realignment of American politics," he said.
Mr Nader won 0.3% of the vote in 2004, when he ran as an independent. In 2000, when he stood as a Green Party candidate, he was blamed by Democrats for taking votes from Al Gore that might have prevented George W Bush's narrow election win.
Thousands of people attended a protest rally held by the libertarian-leaning Dr Paul - who opposes the Iraq war - near the Republican National Convention in Minnesota, last week.
He won no contests in his party's primary election season but raised large amounts of money and support online.