WASHINGTON, United States (AFP) - Gary Johnson, the former Republican governor of New Mexico, on Sunday (May 29) won the presidential nomination of the Libertarian Party, a sliver group hoping to make an outsized impact in this election year.

Johnson came within a half-point of scoring an outright first-ballot victory at the party's nominating convention in Orlando, Florida; a second ballot put him over the top, with 56 per cent.

"I tell the truth, I am not a liar," Johnson told the group, insisting that his frank approach would appeal to disaffected voters and help the long-marginal Libertarians achieve "major-party status."

As a Libertarian, Johnson advocates eliminating the income tax and abolishing the Internal Revenue Service.

A self-made businessman who worked as New Mexico governor to lower taxes and reduce bureaucracy, he pushed for the legalisation of marijuana.

In 2012, he was the Libertarian candidate, garnering 1.2 million votes, the party's best showing ever.

In at least two recent national polls, Johnson scored 10 per cent in hypothetical three-way contests against Donald Trump and Hillary Clinton.

The Libertarians - whose central goal was pithily described by one delegate in Orlando as "minimum government, maximum freedom" - hope to tap into widespread discontent this year with the major-party choices.

Johnson ramped up criticism of one of those choices, the Republican Trump, telling reporters Sunday that the real estate developer was a "racist" because of his description of Mexican immigrants as rapists.

In an interview earlier this month with AFP, Johnson described Trump and Clinton as "the two most polarising figures in American politics today."

He added, "I'm more liberal than Hillary on social issues, and I'm more conservative on fiscal issues than Ted Cruz was," said Johnson, referring to the Texas senator who quit the Republican race early this month.

That, Johnson said, made him "the best of both worlds."

The Libertarians' convention drew far closer media attention than usual, and Johnson told the group that "millions of people are going to be trying to understand what it is to be a Libertarian."

One chart displayed at the convention showed web searches for the party quintupling after Trump became the presumptive Republican nominee.

In balloting carried out on plain index cards, Johnson beat out contenders including Austin Petersen, a businessman and political commentator, and John McAfee, the founder of the antivirus software company who once fled Belize after police sought to question him in a murder case.

The Libertarian convention was to vote separately for its vice presidential nominee.

Johnson said Sunday that the party needed to nominate William Weld, a former Republican governor of Massachusetts, to serve as his running mate, although Weld, a recent convert to Libertarianism, received a cool welcome from many delegates.

"Bill Weld was my role model," Johnson said.

He told delegates and reporters that he did not think he could be elected president without Weld as his running mate.

American political conventions have long been colorful affairs and this has been no exception.

One delegate serenaded the group with a harmonica tune, offering to make it the party's "semi-official" theme song.

Another suggested the party adopt Dobby, a "house elf" from the Harry Potter series, as its official mascot.