Independent presidential candidate Robert F. Kennedy Jr. speaks in Phoenix, Arizona, August 23, 2024. Olivier Touron, AFP/File

WASHINGTON -- US President Donald Trump's embattled health secretary pick, vaccine conspiracy theorist Robert F. Kennedy Jr, passed a crucial test in his confirmation bid on Tuesday as senators voted to advance his nomination to the floor.

With Republicans having a one-seat advantage on the Senate Finance Committee, Kennedy's future hinged on Bill Cassidy, a physician who has clashed publicly with the former Democrat over past unfounded claims linking vaccines to autism.

But Cassidy backed Kennedy after the 71-year-old environmental lawyer was given a strong vote of confidence from Trump, who urged support for the man whom just nine months ago he was calling "one of the most Liberal Lunatics ever to run for office."

"20 years ago, Autism in children was 1 in 10,000. NOW IT'S 1 in 34. WOW! Something's really wrong. We need BOBBY!!!" Trump posted on his website, Truth Social, ahead of the vote.

Cassidy is a rare occasional rebel in the Republican ranks but he faces reelection next year in Louisiana and risked picking up a challenger from the Republican Party's Trumpist far right if he had upset the president.

Kennedy's various controversies are too numerous to itemize but the headline concerns for conservatives are his past support for abortion, his record taking on big business as an environmental lawyer and his 2023 run for president as a Democrat.

Beyond vaccines, Democrats point mainly to sexual misconduct allegations, Kennedy's suggestion that Covid-19 was designed to spare Jews, his linking of school shootings to antidepressants and his alleged mistreatment of animal corpses.

The New York Post, a reliably Trumpist newspaper, wrote a scathing editorial arguing that there was "too much wackiness" in Kennedy's background to trust him with America's health.

READ: Trump's health sec pick RFK Jr faces critical Senate hearing

- 'Conspiracy peddler' -

And Elizabeth Warren, the vice chair of the Senate Democrats, called Kennedy an "anti-science conspiracy peddler who is willing to gamble with American lives," urging senators to reject the nomination.

The Senate can confirm nominees without a committee's endorsement -- it happened in the last Trump presidency with Mick Mulvaney -- but Republican Majority Leader John Thune has cast doubt on how likely this is to happen.

Tulsi Gabbard -- another conspiracy theorist with a long record of publicly opposing US national security policy, including siding with its adversaries -- faces her own key vote with her future also on a knife edge.

Success for Kennedy and Gabbard -- two of the least experienced nominees in modern history -- would once again demonstrate Trump's iron grip on his party, days after controversial Defense Secretary Pete Hegseth's confirmation.

Like Kennedy, Gabbard is running the gauntlet of a party-line vote and just a single Republican no would sink her chances of endorsement by the Senate Intelligence Committee.

READ: Trump says vaccine skeptic RFK Jr will have 'big role' in health care if he wins

READ: Influential podcasts fuel 'harmful' health misinformation

Susan Collins -- considered a swing vote -- announced Monday that she would support the Hawaiian US Army Reserve officer, making fellow Republicans Todd Young and Jerry Moran the only remaining wild cards.

Collins said the one-time lawmaker -- like Kennedy, a former Democrat who once ran for president -- had addressed her concerns over her past support for pardoning NSA leaker Edward Snowden.

The Senate on Monday confirmed Chris Wright, the CEO of fracking company Liberty Energy, to lead the Energy Department, with support from seven Democrats.

Former US congressman Doug Collins of Georgia will be confirmed as secretary of veterans affairs on Tuesday.

Pam Bondi, a replacement for disgraced nominee Matt Gaetz, is expected to be confirmed as attorney general, in the early hours of Wednesday morning at the latest.

© Agence France-Presse