As Bob Barr, a former Republican, reignited his political career by taking the Libertarian presidential nomination at the party’s convention in Denver on Sunday, the political career of Mike Gravel came to a close. It was the former Democrat’s second loss of a party’s presidential nomination in a single year.

“I just ended my political career,” Mr. Gravel said at the party’s convention. “From 15 years old to now, my political career is over, and it’s no big deal.”

Mr. Gravel, 78, spent 12 years in the United States Senate from Alaska, pursuing even then an anti-war agenda and railing against the military industrial complex. His tenure was marked by dramatic procedural actions criticizing the Vietnam War, but his claim to fame came when he tried to release the Pentagon Papers by reading them on the Senate floor. He also waged a one-man filibuster — ended when his colleagues invoked cloture — against the military draft in 1971. He lost his seat in 1980, after a contentious Democratic primary.

When he began his bid for the Democratic nomination last year, he said he was only doing it to promote his National Initiative for Democracy, which would allow citizens to vote on and enact federal legislation.

But then came the first debate, when Mr. Gravel announced that his seven fellow contenders “frighten me.” He also complained that he felt like a “potted plant,” and asked Senator Barack Obama, “Who do you want to nuke?”

Debates provided key exposure for Mr. Gravel, especially since seeing him in person was so difficult: citing security concerns, he refused to release a schedule of his public appearances. So his campaign faced a major setback when networks began barring him from the Democratic cattle calls.

Mr. Gravel said the corporate networks were censoring him because he opposed going to war in Iran. “I’m the only one talking about the military industrial complex,” he said in October, shortly after he did not receive an invitation to a MSNBC debate. “You’d have an interest in cutting my voice out before it become broadly known.”

Mr. Gravel then started holding his own “alternative debates,” aided by a digital video recorder. When he wanted to respond to a question in a real debate, or argue with another candidate, all he had to do was pause the tape and talk — without facing a rebuttal or follow-up.

He found his platform, though, on YouTube, where his videos became instant cult classics. One of his first hits had Mr. Gravel staring at the camera, silently, for more than a minute before throwing a stone in a lake and walking away. Then there was his psychedelic folk-rap, “Power to the People vs. Give Peace a Chance,” which featured the white-haired candidate reciting a slam poem in front of flashing, retro animation.

In March, nearly two years after beginning his bid for the Democratic nod, Mr. Gravel announced that he was changing his registration to Libertarian and that he would pursue the party’s nomination. Sunday’s balloting ended that aspiration, but during his campaigns, Mr. Gravel did win over one key figure: Obama girl.

“You should drop your crush on Obama,” he serenaded the star of the satirical video in a more recent offering by the comedy group Barely Political. “You oughta know that you’re off the chain.”

“Six-foot-two from Alaska, I could give that a shot,” she replied.

“M.C. Gravel-ah hella-copter up in hiz-house,” he rapped.

Just not the White House.

Mr. Gravel could not be reached on Tuesday for comment. But in an e-mail message to supporters, Mr. Gravel’s campaign announced that he would continue to promote his federal referendum initiative, as well as two books he co-wrote. “The Kingmakers,” billed as “an analysis and polemic on corporate media and how it distorts our political dialogue, including Mike’s own campaign,” and “A Political Odyssey,” a story of Mr. Gravel’s career, are both scheduled to be published this summer.

Correction: June 2, 2008 This post originally overstated the impact of Mr. Gravel’s filibuster against the military draft in 1971. It did not lead directly to the compromise that let the draft expire in 1973.