Long-time environmental lawyer and anti-vaccine activist Robert F Kennedy Jr says he will run for United States president as an independent and drop his Democratic primary bid.

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

It is unclear whether Republican support would translate to a general election if Kennedy would also be running against Donald Trump.

Allies of both President Joe Biden and Trump have at times questioned whether Kennedy would be a spoiler against their candidate.

Biden’s allies so far have dismissed Kennedy’s primary campaign as not serious.

Asked for comment on his potential independent run, a Democratic National Committee (DNC) spokesman responded with an eye roll emoji.

Awaiting Kennedy’s remarks on Monday, hundreds of supporters gathered at Philadelphia’s Independence Mall, an apt location for the launch.

Campaign signs teased a potential new slogan: “Declare your independence.”

Among Kennedy’s fans in the crowd were several voters who said they do not identify as Democratic or Republican and view 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 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,” 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.

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

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

“If they jam me, I’m going to look at every option,” he said in September at a New Hampshire barbecue held by Republican former senator Scott Brown.