Compact and Feisty Bob Barr, 59, probably will seek and get the presidential nomination of the Libertarian Party, which convenes in Denver on Memorial Day weekend. Given the recent fund-raising prowess of a kindred spirit—Ron Paul's campaign for the Republican nomination siphoned up $35 million, mostly off the Internet—libertarians are feeling their oats. Come November, Barr conceivably could be to John McCain what Ralph Nader was to Al Gore in 2000—ruinous. Nader was a weak third-party candidate but was the most consequential in American history. He won only 2,882,955 popular votes nationwide (2.7 percent), but 97,488 of them were in Florida, where, because of Nader, George W. Bush won by 537 votes.

The son of a soldier, Barr graduated from high school in Tehran. In 1994, he was elected to Congress as the Republicans, led by another pugnacious Georgian, Newt Gingrich, ended 40 years of Democratic control of the House. Four years later, Barr, a former prosecutor inflamed by charges of perjury and obstruction of justice, was central to the impeachment of Bill Clinton. Since losing his seat in 2002, he has been active in the National Rifle Association and the American Civil Liberties Union, an unusual tandem.

Shane Cory, the Libertarian Party's executive director, knows that directing libertarians is like herding cats—almost a contradiction in terms. But he thinks his party is upwardly mobile. In 2004, its presidential candidate received just 397,265 votes, a mere .32 percent of the national popular vote. The party did best in Indiana (18,058 votes, .73 percent). But in no state was the Libertarian vote larger than the winning candidate's margin of victory. This year, however, Cory thinks the party can far surpass its best national performance—921,299 votes (1.1 percent of the total) in 1980. It has recruited 600 down-ballot candidates around the nation (including Michael Munger, chairman of the political-science department at Duke, who is running for governor of North Carolina) and expects to have 1,500 by Election Day.

The party's immediate challenge is to win ballot access. Barr and Cory say the party almost certainly will be on the ballot in 48 states, and perhaps on West Virginia's, but probably not Oklahoma's. Although Libertarian candidates have been on all 50 several times, the two major parties use laws and litigation to impede ballot access.

In 1968, George Wallace's supporters, with little national organization and negligible financing, got him on all states' ballots on the American Independent Party line. California required 66,000 signatures—not a daunting total but the signatures had to be gathered in 1967, and all signatories had to fill out a two-page legal-size form to register as members of Wallace's new party. More than 100,000 did. Ohio required Wallace supporters to gather 433,000 signatures—in 10 weeks. When that total was surpassed, an Ohio court ruled that Wallace's party was "fictional" because it was a phenomenon of spontaneous combustion. Wallace stopped execrating the U.S. Supreme Court long enough to ask it—successfully—to order Ohio to put him on the ballot.

Wallace had the three traits that, when combined, make a third-party candidate formidable. He had a burning issue (national disorder that he blamed on the civil-rights revolution), a regional base (the South) and a vivid personality. Barr's issues are national. They include limiting government, defending civil liberties during the war on terror, opposing preventive wars and "nation-building," and combating the elephantitis of the presidency. He especially opposes the "unitary theory of the presidency," which he says is: Where the Constitution gives the president power (e.g., national security), no other branch of government has any constitutional authority to limit it.