To say that Kash Patel admires President Trump would be an understatement — and Mr. Patel has seldom been accused of understatement.

His steep, swift ascent from unknown Republican congressional aide to a nominee for F.B.I. director in less than a decade owes much, if not all, to Mr. Patel’s relationship with the president, who rewarded his intense loyalty and perseverance with a succession of senior national security and defense posts during Mr. Trump’s first term.

It was the unflinching fealty he exhibited during Mr. Trump’s turbulent four years out of office that seems to have elevated Mr. Patel, 44, from a supporting player to a leading role (even if Mr. Trump recently quipped that he did not fit his own central-casting image of an F.B.I. director).

In nominating Mr. Patel, Mr. Trump called him a “brilliant lawyer” and an “America First fighter.”

Here’s how Mr. Patel described Mr. Trump at a conservative political conference last year: “We’re blessed by God to have Donald Trump be our juggernaut of justice, to be our leader, to be our continued warrior in the arena.”

Mr. Patel worked the outside game to prove himself to Mr. Trump. He made over 1,000 media appearances (and attended dozens of in-person events) in which he hammered Mr. Trump’s adversaries; wrote a now infamous book in which he singled out 60 perceived enemies for unspecified retribution; published a three-volume children’s series in which he portrayed Mr. Trump as a crowned monarch; and served as a high-volume surrogate on the 2024 campaign trail.

Mr. Patel, a Long Island native, also worked the inside game. He offered national security advice to Mr. Trump; stood by him during the grim days after the F.B.I. search of the president’s Florida estate, Mar-a-Lago, in the summer of 2022; and maximized face time with Mr. Trump and his courtiers in West Palm Beach during the transition period.

But Mr. Trump’s attitude toward subordinates, even ones as enthusiastically supportive as Mr. Patel, tends to be a little diffident. Mr. Trump picked Mr. Patel after the only other serious candidate to lead the bureau, Missouri’s attorney general, Andrew Bailey, failed to impress during interviews, according to people familiar with the situation.

He has occasionally expressed doubts about Mr. Patel’s gravitas, as have many other Republicans — although they have refrained from saying so publicly for fear of incurring Mr. Trump’s wrath.

Mr. Patel has told Republican senators that he will remain independent, faithful to the law and the Constitution, and reform-focused if they back him. So far, it seems to have paid off, although he is walking a thin red line: People close to the nominee believe he has just barely enough votes in the Senate to secure his confirmation, provided his hearing before the Judiciary Committee on Thursday does not go off the rails.

It might. Democrats held their fire when Pam Bondi, Mr. Trump’s pick for attorney general, appeared before the committee, so they could unload on Mr. Patel, whom they have cast as an inexperienced, hyperpartisan Trump sycophant.

“He has neither the experience, the judgment, nor the temperament to head this critical agency,” Senator Richard J. Durbin of Illinois, the ranking Democrat on the committee, said in a statement on the eve of Mr. Patel’s confirmation hearing.

“He has pledged his loyalty to President Trump and promised to weaponize the F.B.I. on President Trump’s behalf,” Mr. Durbin added.