With Mayor Bloomberg out of the presidential race and Ralph Nader’s appeal on the wane, the third-party presidential candidates of the moment may be a former Democratic senator of Alaska, Maurice Gravel, and a former Republican congressman from Georgia, Robert Barr.

No one expects either Mr. Gravel or Mr. Barr to be the next president of America. But as Mr. Nader and another third-party candidate, Patrick Buchanan, showed in Florida in 2000, even a few thousand votes can be enough to swing a close election between the major-party candidates.

“I think it makes great sense for the Libertarians, the Greens, the Independents, and other parties, for a fusion party to rally around a single candidate who can match the Democrats and the Republicans,” Mr. Gravel told The New York Sun yesterday. “If I can get the Libertarian nomination for president, I can be in the race all the way up to November 4th to take on the Democratic and Republican nominees, who are both going to be pro-war and pro-military-industrial complex, all of which I oppose.”

Mr. Gravel spoke a day after he formally joined the Libertarian ranks and renounced his Democratic Party membership. “I’m leaving the Democratic Party because it left me,” he said. “Being a party member doesn’t mean you put the party above principle.”

The Libertarian presidential nomination would offer Mr. Gravel one of the surest routes to getting on the ballot in nearly every state. “Certainly, it would help him,” the publisher of a newsletter tracking such issues, Richard Winger of Ballot Access News, said. “The Libertarian Party nominee was on the ballot in 48 states plus the District of Columbia in 2004 — all 50 in the previous three elections.”

However, Mr. Winger said Mr. Gravel’s sudden transition could rub Libertarians the wrong way. “It just looks terribly opportunist. He should have joined years ago if he was going to do this. He joins the party and comes out the next day and wants the party’s presidential nomination?”

Members of the Libertarian Party are welcoming the decision by Mr. Gravel, who had waged a longshot antiwar candidacy for the Democratic presidential nomination, to join their party, which opposes foreign military adventures by America while backing small government and drug legalization at home. But they are expressing some skepticism of Mr. Gravel’s plan to press his presidential campaign as a “fusion” candidate representing the Libertarians and other third parties.

A top Libertarian Party official and the editor of ThirdPartyWatch.com, Steven Gordon, said Mr. Gravel’s proposal for a fusion candidacy was intriguing, but probably wouldn’t fly. “I think it would be a very difficult sell,” Mr. Gordon said, though he noted that a Senate candidate in Maryland, Kevin Zeese, managed to win concurrent nominations in 2006 from the Libertarians, the Greens and the Populist Party. “It’s not out of the realm of possibility, but I’d say it’s highly improbable.”

One challenge for Mr. Gravel is that many Libertarians are hoping that Mr. Barr, who attracted national attention as a member of the House Judiciary Committee during President Clinton’s impeachment, will throw his hat into the presidential ring.

“I’ve not met Senator Gravel, but certainly I’m delighted that he joined the Libertarian Party apparently for many of the same reasons that caused me to leave the Republican Party,” Mr. Barr said in an interview yesterday. Mr. Barr said third parties share a common interest in getting access to ballots and debates, but he sounded dubious about Mr. Gravel’s talk of a “fusion” campaign. “I’m not sure that policy or agenda has really been well thought through,” Mr. Barr said.

While Libertarians and Greens usually oppose both the Iraq War and the war on drugs, they diverge sharply on issues such as trade, the environment and health care, with Greens favoring a major role for government and Libertarians preferring a laissez faire approach.

Mr. Gravel said he thought he could bridge that divide. “I know government can be the most oppressive agency in human experience. But I don’t fear government,” he said.

Mr. Barr, a conservative who gained attention for teaming up with the American Civil Liberties Union against surveillance measures, said he would not “dally” in making a decision about whether to mount a Libertarian bid for president, but he did not give a specific time frame. The party’s convention is set for May 22 to 26 in Denver.

Whoever wins the Libertarian nomination is likely to have company on the ballot in most states. A perennial presidential hopeful, Mr. Nader, is running again this year. “He’s doing very well on ballot access,” Mr. Winger said. A Libertarian who ran in the Republican primary, Rep. Ron Paul of Texas, has said he’s not interested in making a third-party bid.

While Mr. Gravel argued for unity, Mr. Winger said third parties might have the best chance of getting into debates this fall if there are credible candidates from both the left wing, such as Mr. Nader, and the right, such as Mr. Barr. That could “create a balance” where the Democratic and Republican nominees feel they won’t be disadvantaged by letting others on the stage, the newsletter editor said.