Longtime environmental lawyer and anti-vaccine activist Robert F Kennedy Jr has said he will run for US president as an independent and drop his Democratic primary bid.

Mr Kennedy, a member of one of the most famous Democratic families in US politics, was running a longshot primary bid and has better favourability ratings among Republicans than Democrats.

It is unclear whether Republican support would translate to a general election when Mr Kennedy would be running against Donald Trump. Both Biden and Trump allies have questioned whether Kennedy would be a spoiler against their candidate.

President Joe Biden’s allies so far have dismissed Mr Kennedy’s primary campaign as “unserious”. Asked for comment on his potential independent run, a Democratic National Committee spokesman responded with an eye roll emoji.

Awaiting Mr Kennedy’s remarks on Monday, hundreds of supporters gathered at Philadelphia’s Independence Mall. Campaign signs teased a potential new slogan: “Declare your independence”.

Among Mr Kennedy’s fans in the crowd were several voters who said they do not identify as Democratic or Republican and view Mr Kennedy as a truth teller and a breath of fresh air.

“He tells it how it is,” said Julia Hill, a 23-year-old student from New Jersey. “He doesn’t sound like a politician.”

Other supporters, such as Brent Snyder, a disabled veteran from south Philadelphia, said they had voted for Mr Trump in the past but were looking for a change.

“The last couple years I’ve been noticing the Republican Party’s been going a way I didn’t like,” Mr Snyder said. “Not that I agree with everything that’s happening to Trump, but I think right now he has more baggage than his country needs. The division right now is just terrible. We need someone to bring both sides together to make us work.”

Monday’s announcement comes less than a week after the progressive activist Cornel West abandoned his Green Party bid in favour of an independent White House run.

Meanwhile, the centrist group No Labels is actively securing ballot access for a yet-to-be-named candidate.

Mr Kennedy has spent weeks accusing the Democratic National Committee (DNC) of “rigging” the party’s primary against him and threatening that he might need to consider alternatives.

In campaign emails and videos, he blasted the DNC’s decision not to host debates between Mr Biden and other candidates and railed against the committee’s plan to give South Carolina rather than Iowa or New Hampshire the leadoff spot on the primary calendar this election cycle.

“If they jam me, I’m going to look at every option,” he said in September.

Far-right and anti-vaccine influencers close to Mr Kennedy have also sent strong signals on social media suggesting he should or will leave the Democratic Party.

Last month, Joseph Mercola, an influential anti-vaccine doctor who is allied with Mr Kennedy, ran a poll on X, formerly known as Twitter, asking if Mr Kennedy should quit the party.

While Mr Kennedy has long identified as a Democrat and frequently invokes his late father, Senator Robert F Kennedy, and his uncle former president John F Kennedy on the campaign trail, he has built close relationships with far-right figures in recent years.

He appeared on a channel run by the Sandy Hook conspiracy theorist Alex Jones and headlined a stop on the ReAwaken America Tour, the Christian nationalist road show put together by Mr Trump’s former national security adviser Michael Flynn.

He also has gained support from some far-right conservatives for his fringe views, including his vocal distrust of Covid-19 vaccines, which studies have shown are safe and effective against severe disease and death.

Mr Kennedy’s anti-vaccine organisation, Children’s Health Defence, currently has a lawsuit pending against a number of news organisations, among them The Associated Press, accusing them of violating antitrust laws by taking action to identify misinformation, including about Covid-19 and Covid-19 vaccines.

Mr Kennedy took leave from the group when he announced his run for president but is listed as one of its lawyers in the lawsuit.