PHOENIX (AP) — Robert F. Kennedy Jr. apologized Tuesday after a video was posted online showing part of a private phone call between the independent presidential candidate and Republican former President Donald Trump.

The video shows Kennedy listening on a speakerphone as Trump shares disproven claims about childhood vaccines, an issue that has helped Kennedy amass a loyal following among people who reject the scientific consensus that the benefits of vaccines far outweigh the risk of rare complications. Trump also appears to pitch Kennedy on endorsing his campaign.

“I would love you to do so,” Trump tells Kennedy. “And I think it’ll be so good for you and so big for you. And we’re going to win.”

Kennedy says little in the portion of the conversation that was leaked, which begins while Trump is already speaking about vaccines.

“When President Trump called me I was taping with an in-house videographer,” Kennedy wrote on the X platform. “I should have ordered the videographer to stop recording immediately. I am mortified that this was posted. I apologize to the president.”

The video was first posted by Kennedy’s son, Robert F. Kennedy III, who said it was recorded Sunday, a day after Trump was shot at a rally in Pennsylvania and a day before the start of the Republican National Convention. It was deleted a short time later but copies continue to circulate on social media.

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A spokesperson for Kennedy, Stefanie Spear, said Monday he is not dropping out. His campaign has focused on the arduous task of getting on the ballot in all 50 states without the support of a political party, which requires considerable time and money.

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Allies of both Trump and Democratic President Joe Biden worry about how Kennedy’s campaign will affect their own White House prospects. Third-party candidates rarely get more than a few percentage points of the vote, but Democrats blame Green Party candidates in 2000 and 2016 for tipping the elections toward Republicans.

Kennedy has used nontraditional platforms including podcasts and YouTube to build a following with younger voters and those who distrust institutions, groups Trump hopes to bring into his fold. Democrats worry that Kennedy will pick up some of the anti-Trump voters they hope would instead go to Biden, helping the former president to win.

In his call with Kennedy, Trump discusses the assassination attempt against him and the phone call he received afterward from Biden, which he said “was very nice.” He likened the feeling of the bullet slicing his ear to “the world’s largest mosquito.”