WASHINGTON (Reuters) - Former Republican presidential candidate Ron Paul urged voters on Wednesday to support third-party candidates and said he had turned down a request to endorse John McCain, his party’s nominee.

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At a news conference with Ralph Nader and other third-party presidential candidates spanning the political spectrum, Paul said voters in the November 4 election should look beyond “the lesser of two evils” -- McCain and Democratic nominee Barack Obama.

But Paul’s effort to sound a note of unity among candidates outside the mainstream was undermined by Libertarian nominee Bob Barr, who refused to appear at the event.

“I’m not interested in third parties getting the most possible votes. I’m interested in Bob Barr as the nominee for the Libertarian Party getting the most possible votes,” Barr said at a separate news conference in the same building less than an hour after Paul’s event.

Paul, a Texas congressman who once ran as the Libertarian presidential candidate, drew millions of votes this year and raised a surprising $35 million in the Republican primary race but failed to win any of the state-by-state nominating contests.

He dropped out of the race in June and continued to cultivate his ardent following. He drew 10,000 people to a rally in Minneapolis last week as Republicans met in nearby St. Paul to officially nominate McCain.

He is running for re-election to Congress as a Republican.

Paul has declined to endorse any presidential candidate, although he has spoken favorably of Barr and Constitution Party candidate Chuck Baldwin, who appeared at the news conference along with Nader and Green Party candidate Cynthia McKinney.

The McCain campaign reached out on Tuesday for the first time to seek his endorsement but he declined, Paul said.

“The argument was he would do a little less harm than the other candidate,” Paul said.

The McCain campaign did not return a call seeking comment.

That same day, Barr offered the Libertarian vice presidential spot to Paul. Barr said he had not yet heard back, but did not expect him to accept.

“Dr. Paul has zero interest in that,” his spokesman, Jesse Benton, said.