He's an environmentalist and conspiracy theorist, an anti-establishment figure from one of America's most storied political families, a virile 70-year-old with a host of health issues.

It can be hard to get a handle on Robert F Kennedy Jr, the nephew of former president John F Kennedy, but one thing's for sure: his bid for the White House can't be easily dismissed.

The independent candidate has fallen short of qualifying for this week's presidential debate, but his polling numbers — up to around 15 per cent — suggest his candidacy could be a deciding factor in who takes the Oval Office in 2024.

That's because the winner of the expected Biden–Trump rematch could be determined by a relatively small number of votes in a handful of swing states. If RFK Jr siphons significantly more support from one of the main candidates than the other, it could make all the difference.

A famous name doesn't get you on the ballot

As America endures the year's first heatwave, Colin McEvers is sweating in his shirt and tie in downtown Annapolis, Maryland, trying to recruit passers-by to support RFK Jr's presidential bid.

"Hi guys, how are you doing today?" the towering teenager says, as all 6 foot, 10 inches of him strides towards a couple in their early 40s.

"Sorry to bother you. My name's Colin.

"I'm working for the RFK Jr campaign. We're just trying to get some signatures to get an independent on the ballot for Maryland... as another option besides Biden and Trump, basically."

Third-party candidates are not guaranteed to get their names on ballots in US elections. The requirements vary from state to state. Here in Maryland, RFK Jr needs 10,000 verified names on a petition.

McEvers, the state campaign co-ordinator, says he's personally collected more than 3,000.

With polls showing a majority of voters have unfavourable views of both Joe Biden and Donald Trump, the case for a third option is hitting home.

"Trump and Biden are both often off the rails," says Karl Klinghard, who's visiting from Tennessee and says he'll "probably" vote for RFK Jr in this November's election.

"I think RFK couldn't do any harm," he says.

"I like his environmental aspect, and I think he's got a lot to offer, you know just from his background and private industry and all those things."

A life marked by tragedy and conspiracy 

RFK Jr was nine years old when his uncle, then-president John F Kennedy, was assassinated in 1963.

Five years later, Robert F Kennedy — JFK's brother and the current candidate's father — was also shot dead while he was seeking the Democratic presidential nomination.

RFK Jr, one of the late Bobby Kennedy's 11 children, has long espoused the view that the death of his uncle was orchestrated by the CIA.

He's also called for a new investigation into the murder of his father, arguing the evidence suggests the man serving a life sentence for the killing, Sirhan Sirhan, did not fire the fatal shots.

Perhaps these experiences helped shape RFK Jr's deep distrust of government, institutions and the media, a worldview that has seen him embrace vaccine conspiracy theories.

He's been a vocal advocate of the now-debunked theory that vaccines in children are linked to autism.  During the pandemic, he suggested COVID may have been engineered to target certain ethnic groups, and appeared to back a theory that Bill Gates was using the vaccine to control people via microchip.

"What was yesterday's conspiracy theory is today's truth," says Josh Mazer, Maryland Team Kennedy campaign co-ordinator.

"He's simply the most intelligent person in any room I've ever walked into."

In recent campaign appearances, RFK Jr has appeared to tone down his vaccine rhetoric, but still rails against COVID public health measures. He described them as "the greatest restriction on individual liberties this country has ever known" during a recent speech at the Libertarian Party convention in Washington DC.

The speech also touched on one of RFK Jr's core policy proposals: slashing military funding and getting out of foreign wars.

"We have forfeited our moral authority because we have supported dictators and corrupt regimes and subverted democracy around the globe," he told the crowd.

Some of the biggest cheers came when he said that, on his first day in office, he would drop all charges against Julian Assange and pardon Edward Snowden.

"Free Assange, free Assange," audience members chanted.

As news broke on Monday, local time, that Mr Assange had reached a plea deal with US authorities, RFK Jr suggested that a monument be built to the Australian to educate the American public about the importance of free speech.

Describing Mr Assange as a “generational hero”, RFK Jr lamented the terms of the deal, which he said meant the US had succeeded in criminalising journalism.

While the Libertarians didn't vote him in as their 2024 candidate, he had plenty of fans in the room.

"I like the fact that he's not a politician," said Nancy Lou Little, a veterinarian who shares his scepticism about vaccines, America's health regulation bodies and big pharma.

"When I first heard him interviewed, I started crying because he was talking about our health problems. And I've been waiting 35 years for somebody to talk about those."

Spoiler alert for Biden and Trump

RFK Jr began his bid as a Democratic candidate, challenging Joe Biden for the nomination.

Last October, he announced he was running as an independent.

His "America First" tendencies and distrust of big government potentially appeals to Republican voters.

Yet his credentials as an environmental lawyer who helped clean up the Hudson River in New York, and his rejection of a ban on TikTok, could see win him support from younger, more left-leaning voters.

That's the thing with RFK Jr — you can really choose your own adventure.

"I'm not sure anyone knows where he stands because he is all over the place ideologically," says Bernard Tamas, a professor of political science at Valdosta State University in Georgia.

"A big part of his appeal is the fact that Americans are very disenchanted with the political system right now and with both candidates."

Colin McEvers, the campaigner in Maryland, likes what he sees of the candidate's plans to make first home loans more affordable by guaranteeing government-backed mortgage interest rates.

"I want to own a home in the future, I want to build a family, and it's very hard for people of my age range to do that."