Washington DC - Armed with the most storied surname in US politics, Robert F. Kennedy's wildcard shot for the presidency holds a clear and present danger for Joe Biden's hopes of a second term in the White House.

The environmental lawyer and conspiracy theorist is boasting double-digit support, and polling suggests that independent candidate "RFK Jr" is hurting the president more than Republican challenger Donald Trump.

Democrats have learned to fear long-shot outsiders after George W. Bush and Donald Trump won tight elections in 2000 and 2016, buoyed by Green Party candidates leeching votes from Al Gore and Hillary Clinton.

"Hyper-polarization is the reason that a third-party candidate with name recognition is such a threat at this time," said Donald Nieman, a professor and political analyst at New York state's Binghamton University.

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"There are only six or seven truly competitive states, and some of those states will be decided by as few as 10-20,000 votes. So anything that siphons a group of usually reliable voters away could be a deciding factor."

Kennedy's popularity – he is pulling around 10% in election polling averages – complicates the strategy for the Biden campaign, which is seeking to make November's vote a binary choice between the president and Trump.

Kennedy, a vaccine skeptic who has spread misinformation downplaying the 2021 insurrection at the US Capitol, has yet to secure ballot access in most of the country.

But says he has enough support to compete in six states and is aiming to raise one million signatures to qualify in all 50.

"Kennedy represents a real threat to the Biden campaign in key swing states," said Charlie Kolean, the chief strategist at conservative political consultancy RED PAC.

"He's polling exceptionally well with independents and the younger generation – groups that have traditionally voted for Biden."

Progressive super PACs and the Democratic National Committee have ramped up anti-Kennedy attacks, sending mobile billboards out to his events and pursuing legal efforts to keep him off the ballot.

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For all their fears, a conservative rather than a – businessman, Ross Perot – remains the most successful wildcard in modern history, snagging 19 percent in 1992 to deny George HW Bush's victory over Bill Clinton.

Kennedy's conspiratorial views and opposition to Ukraine aid once made him a darling of Trump's "Make America Great Again" (MAGA) movement, but Trump has dismissed the notion that Kennedy could be another Perot.

"He is Crooked Joe Biden's Political Opponent, not mine. I love that he is running!" Trump posted recently on his social media app.

Kennedy (70) started his White House bid as a Democrat but exited the primary after complaining of bullying tactics by Biden allies seeking to keep him out of contention.

Keith Nahigian, a Bush aide in the Perot-assisted 1992 defeat to Clinton, believes Biden made a "gigantic error" in alienating Kennedy, a candidate the Democrat could have swept out of the way in the primary contest.

"I think they might lose the presidency because they didn't think through the strategy in the beginning of this race," he told AFP.

A veteran of six presidential campaigns, Nahigian cautions against viewing 2024 as another 1992, however, pointing out that billionaire Perot could buy access and publicity that Kennedy cannot afford.

Kennedy moved some way to closing the funding gap – and further perturbing Democrats – by naming as his running mate Nicole Shanahan, a Californian former Democratic donor with deep pockets.

Nahigian described Shanahan, an attorney and philanthropist, as "very far left" and unlikely to peel off Trump supporters. But he warned Republicans and Democrats alike against complacency.

"Independent candidates make a big impact on an election and it doesn't matter if you're a sitting president, if you're a former president – you have to take it seriously," Nahigian said.

"You've seen the last couple of weeks both Trump and Biden really start attacking RFK because they now see him as a viable threat, especially to Biden in these key states where he is losing right now."