Libertarians, who believe in limited government and individual freedom, blame Trump, a Republican, for rushing through the creation of a COVID-19 vaccine when he was US president and for not doing more to stop public health restrictions on the unvaccinated during the pandemic.
When Trump took to the stage in Washington, there were loud boos and jeers. A smaller section of the crowd, Trump supporters, cheered him.
Shortly before he appeared, one Libertarian Party member shouted: "Donald Trump should have taken a bullet!"
Trump's campaign did not immediately respond to a request for comment about the hostile reception.
Trump, who was US president between 2017 and 2021, immediately mentioned the total 88 felony charges he faces in four federal and state prosecutions.
"If I wasn't a libertarian I am now," he said.Â
Trump denounced the administration of US President Joe Biden, his challenger in the November 5 election rematch, and Biden's fellow Democrats as being part of a "rise in left-wing fascism".
Trump was trying to appeal to libertarians, who have more in common with Republican policy positions than Democrats on issues including taxes and the size of government, in what is expected to be a closely fought election.
He added: "We should not be fighting each other." He asked libertarians to work with him to defeat Biden, an appeal which was greeted by many boos, although the vast majority in the crowd were fiercely opposed to Biden and his administration.
Libertarians garnered only 1.2 per cent of the national vote in 2020, or about 1.8 million votes, but November's election could be decided by just tens of thousands of votes in a handful of battleground states, so Trump is seeking to peel away some libertarian support.
Trump's appearance at the libertarian gathering, unusual for a Republican White House candidate, also signalled how seriously he and his campaign take the threat of third-party candidate Robert F. Kennedy Jr., who has long opposed vaccines and mandates, and who spoke to the convention on Friday.
Trump has been ramping up attacks on Kennedy, who is running as an independent, recently calling him a "fake" anti-vaccination proponent.
Opinion polls suggest Kennedy will siphon votes away from both Trump and Biden, but it is not yet clear which of the major-party candidates will be hurt more by Kennedy's long-shot White House bid.
Opinion polls suggest Robert F Kennedy Jr will siphon votes away from both Trump and Biden. (AP PHOTO)
Libertarian Party organisers said Biden was also invited to speak to the convention but he declined to attend.
"The Libertarian Party can make a big difference. If we unite we will be unstoppable," Trump said to a mix of applause and jeers.
Trump said he was a "libertarian without even trying to be one," and that the Libertarian Party should endorse him, another line greeted by boos and jeers.
Undeterred, Trump poked fun at the crowd, saying if they did not back him, they would continue to garner just a tiny portion of voter support in national elections.
He pledged to put a libertarian in his cabinet if he wins the election, which was met by cries of "bulls***!"
Trump did get huge applause for one promise. A rallying cry for libertarians is the case of Ross Ulbricht, who is serving a life sentence for creating and operating the website Silk Road, which allowed users to secretly buy and sell drugs and other illegal products.
