Washington DC - Independent White House hopeful Robert F. Kennedy Jr. on Friday accused Joe Biden and Donald Trump of trampling on the US constitution as he launched a trenchant defense of Americans' civil liberties.

In a guest speech in front of a small crowd at the Libertarian Party's national convention in Washington, the anti-vaccine activist berated the president and his Republican predecessor over pandemic-era health curbs and threats to free speech.

"If you treasure freedom, you'd better elect leaders who are inspired by it and who will wield it to inspire others, leaders who believe in freedoms and hold the Bill of Rights in reverence," Kennedy said in a speech delivered to a hall with dozens of empty seats.

"I'm sorry to say that neither President Trump nor President Biden passed this critical examination. Neither of them upheld the Constitution when it really counted."

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After leaving the Democratic Party, Kennedy, the scion of America's most famous political clan, considered joining the Libertarian Party but ultimately opted to run for president as an Independent.

The environmental lawyer attacked Trump over Covid lockdowns and enforced social distancing and faulted Biden for vaccine mandates and "colluding with the FBI" to coerce Big Tech companies to censor the speech of Americans.

He also went after Trump for the "persecution" of WikiLeaks founder Julian Assange, who is fighting extradition to the US over alleged national security breaches.

Kennedy said he would drop the case against Assange and pardon whistleblower Edward Snowden on his first day in office.

The environmental lawyer is in third place and averaging just over 10% in polls, including Trump, Biden, and two minor candidates.

Kennedy challenged Trump to a debate at the convention, in an open letter published on X that pointed to data from polling firm Zogby showing him beating Trump and Biden in hypothetical two-way match-ups.

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The Independent is officially on the ballot in seven states so far – including the crucial battleground state of Michigan – and says it has collected enough signatures for access in eight more.

Trump, who is due to deliver the keynote speech at the four-day convention on Saturday, has been escalating attacks against Kennedy, underscoring the threat he believes the nephew of John F. Kennedy could pose to his campaign.

But Trump focused on Biden in a statement previewing his appearance at the event in a plush downtown Washington hotel, calling his rival "the worst president in the history of the United States by far."

"If Libertarians join me and the Republican Party, where we have many libertarian views, the election won't even be close," he said. "We cannot have another four years of death, destruction, and incompetence."

Biden was also invited to speak at the convention, but officials say his team didn't respond.