Three of President Trump’s most contentious picks to lead government agencies will appear in Senate confirmation hearings on Thursday, with the fate of their nominations hanging on the votes of a handful of Republican senators.

Robert F. Kennedy Jr., the former independent presidential candidate who also threw his support behind Mr. Trump, will face his second hearing before a Senate vote, after a grilling on Wednesday on his views on vaccines and abortion.

Kash Patel, Mr. Trump’s F.B.I. pick, has promised to reshape the bureau by firing its top officials and has published a list of Trump enemies. And Tulsi Gabbard, a former Democratic congresswoman who left her party and embraced Mr. Trump, has been nominated to oversee the nation’s intelligence agencies.

Here’s what to know about the candidates.

Kash Patel, F.B.I. director nominee

Mr. Patel, once an unknown Republican congressional aide, has swiftly risen in less than a decade — in large part because of his personal relationship with President Trump. His fealty to the president and his embrace of right-wing conspiracy theories are expected to be subjects of scrutiny at his confirmation hearing.

A right-wing pugilist, Mr. Patel has repeatedly undermined the F.B.I.’s work in public statements, including its prosecution of those charged in connection with the Jan. 6, 2021, attack on the Capitol. He has also sought to rewrite the history of that attack, pinning the blame for the riot on former Speaker Nancy Pelosi.