By Timothy Pratt

LAS VEGAS (Reuters) - The U.S. Libertarian Party on Saturday chose former Republican New Mexico Governor Gary Johnson as its presidential candidate in the November 6 election.

Johnson, who announced in December he would run for president as a Libertarian, won 70 percent of the votes cast by 632 delegates at the party's convention in Las Vegas, Libertarian Party spokesman Stephen Gordon said.

Johnson, who became the front-runner after winning straw polls at 12 previous state debates, beat Air Force veteran R. Lee Wrights, after the field narrowed to two candidates from six at the start of the convention on Thursday.

"I am very humbled. This is just the start," Johnson told Reuters on Saturday. The party's philosophy is "minimum government, maximum freedom."

Third parties have traditionally fared poorly in the two-party U.S. political system long dominated by Republicans and Democrats.

The Libertarians' best presidential showing came in 1980 when nominee Ed Clark won 921,128 votes or 1.1 percent. In the 2008 election, party nominee Bob Barr, a former Republican congressman, got 523,686 votes or 0.4 percent.

Johnson, 59, is running on a platform that includes slashing government spending to balance the federal budget by 2013, as well as ending the war on drugs, beginning with the legalization of marijuana.

Delegates said their preference for Johnson stemmed from his experience as governor from 1995 to 2003, which gave him a greater chance of success in a national election. Wrights had no prior political experience.

Democratic President Barack Obama is seeking re-election in the November election. Former Massachusetts Governor Mitt Romney is the presumptive Republican nominee.

(Editing by Tim Gaynor and Peter Cooney)