Independent White House candidate Robert F Kennedy Jr has joined the Republican nominee, Donald Trump, on stage at a rally in Arizona after dropping out of the race and endorsing the former US president.

Mr Kennedy, 70, a Democrat for most of his life and the scion of the Kennedy dynasty, said the principles that had led him to leave the party had now compelled him “to throw my support to President Trump”.

He said in a press conference in Phoenix, Arizona, on Friday that he would seek to remove his name from the ballot in 10 battleground states.

Trump praised Mr Kennedy as “phenomenal” and “brilliant” as he welcomed him on stage at the rally later in Glendale. Democratic rival Kamala Harris said she would try to “earn” the support of Kennedy voters.

With November’s election looming, Mr Kennedy’s polling has slumped from a high of double figures as funds and national coverage dried up.

The son of US Senator Robert F Kennedy and nephew of President John F Kennedy, he is from the most famed family in Democratic politics.

Before welcoming RFK Jr to the stage on Friday, Trump promised, if elected, to release all remaining documents relating to the 1963 assassination of President Kennedy.

Mr Kennedy’s decision to back a Republican for the White House has outraged his relatives, who previously condemned his invocation of the family name in a Super Bowl ad back in February.

Kerry Kennedy, his sister, said his support for Trump was a “betrayal of the values that our father and our family hold most dear. It is a sad ending to a sad story.”

“This decision is agonising for me because of the difficulties it causes my wife and my children and my friends,” Mr Kennedy said on Friday.

“But I have the certainty that this is what I’m meant to do. And that certainty gives me internal peace, even in storms.”

He is married to Cheryl Hines, the star of HBO comedy Curb Your Enthusiasm. She posted on X, formerly Twitter, that she deeply respected her husband’s decision to suspend his campaign. She did not comment on his endorsement of Trump.

Mr Kennedy told reporters on Friday that Trump’s insistence he could end the war in Ukraine by negotiating with Russia “alone would justify my support for his campaign”.

“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 he would remove his name from 10 states where his presence would be a “spoiler” to Trump’s effort. He has already withdrawn from the battleground states of Arizona and Pennsylvania.

But it is too late for him to pull out from the swing states of Michigan, Nevada and Wisconsin, election officials told AP news agency.

Mr Kennedy said he had launched his campaign in April 2023 “as a Democrat, the party of my father, my uncle… the champions of the Constitution”.

But he left because “it had become the party of war, censorship, corruption, big pharma, big tech, big money”.

He blamed his decision to suspend his campaign on “media control” and his former party’s efforts to thwart his run, adding: “In my heart I no longer believe I have a realistic path to victory in the face of relentless and systematic censorship.”

Mr Kennedy hovered around 14% – 16% in polls at his most popular. However, his ratings have slumped to single digits since Ms Harris became the Democratic nominee.

He said in his press conference that he had offered to work with Ms Harris and her bid for the White House.

Democrats sounded unfazed by 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.

Mr Kennedy’s campaign became synonymous with the anti-vaccine movement as he frequently touted his leadership of the Children’s Health Defense organisation, formerly known as the World Mercury Project.

In recent weeks, Mr Kennedy recounted how he dumped a dead bear cub that had been hit by a car in New York’s Central Park in 2014 as a joke.

Earlier in his campaign, it was revealed that he had suffered from a brain parasite over a decade ago which caused severe memory loss and brain fog.

His announcement capped days of rumour that Mr Kennedy offered to endorse Trump to secure a role in his next administration.

Trump told CNN earlier this week he would “certainly be open” to Mr Kennedy playing a role, while Trump’s son, Donald Trump Jr, said he would be suited to “blow up” a federal department.

Merrill Matthews, a resident scholar with the conservative Institute for Policy Innovation, told the BBC that Mr Kennedy’s decision highlighted the two-party system in the US and “how difficult it is to get new ideas and fresh people into the process”.

Mike Wendling also contributed to this report.