Washington DC - Donald Trump's picks to lead the US intelligence community and top law enforcement agency were assailed over their lack of experience and past judgment calls Thursday as the Republican president's most contentious cabinet nominees faced showstopping Senate confirmation hearings.

Tulsi Gabbard, tapped for director of national intelligence, sat before the Senate Intelligence Committee for the most consequential confirmation hearing to date, while Kash Patel was questioned on his ambitions to head the FBI.

Gabbard, a former Hawaiian congresswoman who ran for president as a Democrat in 2020, is considered Trump's most vulnerable cabinet-level nominee, and her grilling marked the biggest test of his sway over Senate Republicans since he took office.

She is regarded with suspicion over her past support for NSA leaker Edward Snowden, seen on both sides of Congress as having imperiled national security.

Donald Trump Trump hits critic Mark Milley with new punishment from the Pentagon

She also faced questions over her lack of national security experience, her 2017 meeting with now-deposed Syrian strongman Bashar al-Assad, and her peddling of Russian propaganda, particularly false conspiracy theories about the Ukraine war.

Just one Republican "no" vote would stop Gabbard's nomination from making it to the Senate floor with a favorable report, and the party's leadership has indicated that she wouldn't get a vote without committee support.

Tom Cotton, the panel's Republican chairman, said he was "dismayed" by attacks on Gabbard's patriotism and loyalty, pointing to her two-decade military career and five FBI background checks that he said showed her to be "clean as a whistle."

But Mark Warner, the top Democrat, argued that foreign allies may not be able to trust Washington with their secrets if Gabbard is put in charge of the constellation of 18 intelligence agencies.

In his opening statement he said the US intelligence mission "is all predicated on trust – trust that our allies will protect each other's secrets."

"It appears to me you have repeatedly excused our adversaries' worst actions and instead often blame them on the United States and those very allies," he added.

But Gabbard hit back, arguing that her critics were upset that "I refuse to be their puppet" and saying that Trump won a clear mandate to end the "weaponization and politicization" of the intelligence community.

Donald Trump Panama president makes final call on canal negotiations amid Trump threats

She acknowledged that Snowden had broken the law by leaking highly classified information in 2013 but repeatedly refused, under questioning from both sides, to call him a traitor.

"I'll begin by leading by example, checking my own personal views at the door and committing to delivering intelligence that is collected, analyzed, and reported without bias, prejudice, or political influence," she told the panel.

On a day of drama on Capitol Hill, there were fiery exchanges between Democrats on the Senate Judiciary Committee and FBI director-designate Kash Patel, although he appears to be on a surer footing than Gabbard.

Democrats argued that he is an unrepentant conspiracy theorist and brought up a list of 60 supposed "deep state" actors – all critics of Trump – he included in a 2022 book, whom he said should be investigated or "otherwise reviled."

Senator Dick Durbin, the panel's top Democrat, said Patel had "neither the experience, the temperament nor the judgment to lead" the FBI.

Patel has denied that he has an "enemies list," and told the committee he was merely interested in bringing lawbreakers to book.

In one unexpected exchange, he distanced himself from at least some of Trump's pardons of hundreds of criminals convicted of violence over the 2021 Capitol insurrection.

"I do not agree with the commutation of any sentence of any individual who committed violence against law enforcement," he said.

At the same time, Robert F Kennedy Jr. was appearing for a second hearing on Capitol Hill, a day after coming under attack from Democrats over his promotion of vaccine misinformation and his sudden embrace of anti-abortion policies.

Thursday's questioning turned to past allegations of sexual assault from a babysitter who received an apology from Kennedy after claiming he groped her in 1999.

Kennedy denied sexual misconduct, saying the accusation had been "debunked," and added that he texted the alleged victim an apology for "something else."

Trump has chosen Kennedy to lead the Department of Health and Human Services.