Donald Trump appealed for support from the fringe Libertarian Party on Saturday, telling a rowdy crowd that if elected he would free from prison an American man who ran a website that sold millions of dollars in drugs.

The announcement came at a contentious speech -- where the crowd often jeered and booed the ex-president -- at the Libertarian National Convention in Washington, as he seeks support in the November US election against incumbent Joe Biden.

"If you vote for me, on day one, I will commute the sentence of Ross Ulbricht," Trump said, in a line that brought the rowdy crowd over to his side as cheers erupted from the audience.

He also vowed to put a libertarian in his cabinet.

In 2015, Ross Ulbricht was sentenced to life in prison after being convicted of masterminding the Silk Road "dark web" marketplace, which sold US$200 million in drugs to customers across the world.

Ulbricht, who ran Silk Road under the alias "Dread Pirate Roberts" and was also accused of commissioning five murders at a cost of US$650,000, was sentenced to two life sentences for narcotics distribution and criminal enterprise.

His case has become a famous cause in libertarian circles, who have decried the conviction as government overreach and against the principles of free markets.

Trump has previously vowed to impose the death penalty on drug traffickers.

While the promise to free Ulbricht brought cheers, Trump's speech was far from the usual lovefest he presides over at his rallies, which typically attract hardcore supporters.

The conservative was variously booed as he tried to make common cause with the libertarians in the audience, many of whom view his policies as outside their extremely limited scope of what the government should be able to do.

"The Libertarian Party should nominate Trump for president of the United States," he said to a mix of cheers and boos.

"Only if you want to win. Maybe you don't want to win," Trump said as he at times sparred with the audience.

The speech came after Trump rolled the dice on Thursday by making a campaign stop in deeply Democratic New York, seeking to woo Black and Latino voters whose support for Biden has shown signs of faltering.

"If you want to compete for untraditional votes, you have to take risks," Trump advisor Jason Miller told news outlet Semafor.

The Libertarian Party routinely runs minor candidates that promote both popular limited-government ideas like marijuana legalization while simultaneously pushing fringe ideas like abolishing the federal tax collection agency or social security.

Its candidates have never posed serious threats in presidential elections.

If they weren't interested in mixing it up by endorsing Trump, they were free to "keep getting your three percent every four years," he said. (AFP)