Robert F Kennedy Jr will not be invited to this year’s first US presidential debate between Joe Biden and Donald Trump.
CNN, which is hosting the debate, said Thursday that no other candidates met the requirements to join the two major party contenders on stage.
Mr Kennedy failed to garner enough support in recent polls and has not yet qualified for the ballot in enough states to be eligible under rules set out by the broadcaster.
The independent candidate said that his exclusion was against US election laws and hinted at taking legal action.
“Presidents Biden and Trump do not want me on the debate stage and CNN illegally agreed to their demand,” Mr Kennedy said in a statement, calling the move “undemocratic, un-American, and cowardly”.
Mr Kennedy, the son of Robert F Kennedy and nephew of President John F Kennedy, had previously complained to the US Federal Election Commission (FEC) about the rules and his exclusion from the event.
He is the only third-party candidate who had a real shot at qualifying for the debate stage.
In order to be eligible, candidates were required to be on the ballot in enough places to have a theoretical chance at winning the presidency – in other words, states that add up to 270 or more electoral votes.
Candidates also had to register at least 15% support in at least four major polls.
Mr Kennedy only achieved that level in three recent polls, however.
The ballot question is more complicated. The Kennedy campaign has poured millions into getting on the ballot in all 50 states, each of which has different rules for independent candidates.
The campaign says it has collected enough petition signatures to get on the ballot in 23 states, totalling 310 electoral votes.
However, the campaign acknowledges that only nine states – totalling just 144 electoral votes – have actually confirmed that RFK Jr will appear on the ballot.
The Kennedy campaign has lodged a complaint with the FEC, arguing that Mr Biden and Trump have not yet been nominated by their respective parties and thus are not officially on the ballot in any states.
The campaign has accused CNN of failing to apply objective criteria in deciding who makes the debate stage and have alleged that the broadcast next Thursday amounts to a prohibited “campaign contribution”.
CNN has said that Mr Kennedy’s complaints are baseless. The broadcaster declined to comment when reached by the BBC on Thursday.
The debate announcement came one day after financial reports indicated that Mr Kennedy has struggled to collect donations from supporters and has burned through cash in his efforts to get on state ballots.
The Kennedy campaign collected $2.6m (£2m) in May, down from $10.7m in April – $8m of which was given by Mr Kennedy’s vice presidential running mate, Nicole Shanahan.
Less than $1m of May’s total was given by people giving small donations of $200 (£158) or less.
By contrast, the main contenders are raising much larger sums. The Trump campaign raised $141m in May. President Biden reportedly raised $28m in just one night, at a fundraiser in Los Angeles last week.
“Kennedy’s lack of small donors is really an indication of his lack of support,” said Melissa Smith, a professor at Mississippi University for Women and author of Third Parties, Outsiders, and Renegades: Modern Challenges to the Two-Party System in Presidential Elections.
“The whole thing about the individual small donors is that if somebody will give you $10 or $15, they’ll give you $10 or $15 dollars again in the future,” creating a more sustainable source of funding, Ms Smith said.
“Kennedy seems to be lacking that,” she said.
Kennedy’s overall poll numbers have declined since the start of the year, and despite reaching the 15% threshold in several recent polls, he’s currently at under 10% in both the FiveThirtyEight and RealClearPolitics polling averages.
Even at that level, experts say, Mr Kennedy’s run has the potential to affect the outcome of the expected tight race between Mr Biden and Trump.
What effect he will have is still an open question, though, as opinion polls vary as to whether he will draw more support from Biden or Trump backers – or from people who wouldn’t vote for either major party candidate.