Robert F Kennedy Jr suspends US presidential campaign and backs ...

Independent US presidential candidate Robert F Kennedy Jr has announced he will suspend his election campaign and throw his support behind Republican candidate Donald Trump.

The 70-year-old reportedly filed paperwork on Thursday to remove his name from the ballot in 10 battleground states, but said he would remain a candidate in other states that were unlikely to sway the outcome of the election.

"I no longer believe that I have a realistic path to electoral victory," he said at an address in Phoenix on Friday, local time. 

Mr Kennedy said he had met with Trump and his aides several times and learned they agreed on issues including border security, free speech and ending wars.

"There are still many issues and approaches on which we continue to have very serious differences but we are aligned on other key issues," he said at an address in Phoenix.

"I've made the heart-wrenching decision to suspend my campaign and to support President Trump," he added.

"This decision is agonising for me because of the difficulties it causes my wife and my children and my friends, but I have the certainty that this is what … [I'm] meant to do."

Trump celebrated Mr Kennedy's endorsement at a campaign stop in Las Vegas. 

"I want to thank Bobby. That was very nice," he said. 

"He's a great guy, respected by everybody."

His announcement came after support for him in national polls plunged to about 5 per cent and his campaign funds began to run low.

An environmental lawyer, anti-vaccine activist and son and nephew of two titans of Democratic politics who were assassinated during the turbulent 1960s, Mr Kennedy entered the race in April 2023 as a challenger to President Joe Biden for the Democratic nomination.

Robert Kennedy Jr speaks at the Libertarian Convention in June. (ABC News: Cameron Schwarz)

With voters at the time turned off by both the aging Mr Biden and the legally embattled Trump, interest in Mr Kennedy soared. 

He shifted his plans and decided to run as an independent, and a November 2023 Reuters/Ipsos poll showed Mr Kennedy drawing the support of 20 per cent of Americans in a three-way race with Mr Biden and Trump.

He ran a high-profile advertisement during the February 2024 Super Bowl that invoked his father, former US senator Robert F Kennedy, and uncle, former US president John F Kennedy, and drew outrage from much of his high-profile family.

For a time, both the Biden and Trump campaigns showed signs they were worried that Mr Kennedy could draw enough support to change the election outcome. 

Mr Kennedy built an unusually strong base for a third-party bid, fuelled in part by anti-establishment voters and vaccine septics who had followed his anti-vaccine work since the COVID-19 pandemic. 

But as the race changed quickly in the last two months — with Trump surviving an assassination attempt and 81-year-old Mr Biden bowing to pressure from his own party and passing the campaign torch to Harris — voter interest in Mr Kennedy waned.

Democrats have shrugged off his announcement.

"Donald Trump isn't earning an endorsement that's going to help build support, he's inheriting the baggage of a failed fringe candidate. Good riddance," Democratic National Committee senior adviser Mary Beth Cahill said in a statement.

Kerry Kennedy (right) and other members of the Kennedy family support Joe Biden at a Democratic campaign event in April. (Reuters: Elizabeth Frantz)

A year ago, some would have thought it inconceivable that a member of arguably the most storied family in Democratic politics would work with Trump to keep a Democrat out of the White House. 

Even in recent months, Mr Kennedy has accused Trump of betraying his followers, while Trump has criticised him as "the most radical left candidate in the race".

Five members of the Kennedy family issued a statement on Friday calling his support for Trump "a sad ending to a sad story".

"We want an America filled with hope and bound together by a shared vision of a brighter future, a future defined by individual freedom, economic promise and national pride," read the statement, posted by his sister Kerry Kennedy on X. 

"We believe in Harris and Walz. Our brother Bobby's decision to endorse Trump today is a betrayal of the values that our father and our family hold most dear."

Trump mega-donor Timothy Mellon, an 82-year-old banking heir, has given millions of dollars this election cycle to support Mr Kennedy, and $US75 million ($110 million) to support Trump, according to Federal Election Commission filings through to July 31.

Trump is holding a rally later today in Arizona, which Mr Kennedy is speculated to attend. 

ABC/wires