WASHINGTON — President-elect Donald Trump said Thursday he will nominate former Florida Attorney General Pam Bondi to lead the Justice Department, turning to a longtime ally after his first choice, Matt Gaetz, withdrew his name from consideration amid scrutiny over sex trafficking allegations.

Ms. Bondi has been an outspoken defender of Trump. She was one of his lawyers during his first impeachment trial, when he was accused — but not convicted — of abusing his power as he tried to condition U.S. military assistance to Ukraine on that country investigating then-former Vice President Joe Biden. And she was among a group of Republicans who showed up to support Trump at his hush money criminal trial in New York that ended in May with a conviction on 34 felony counts.

“For too long, the partisan Department of Justice has been weaponized against me and other Republicans — Not anymore,” Trump said in a social media post. “Pam will refocus the DOJ to its intended purpose of fighting Crime, and Making America Safe Again.”

Mr. Gaetz stepped aside amid continued fallout over a federal sex trafficking investigation that cast doubt on his ability to be confirmed as the nation’s chief federal law enforcement officer. Mr. Gaetz, who passed the bar but barely worked as a lawyer, had very little relevant experience for the job. Ms. Bondi comes with years of legal work under her belt and that other trait Trump prizes above all: loyalty.

The hasty withdrawal by Mr. Gaetz and quick pivot to Ms. Bondi were the latest examples of Trump’s tumultuous decision-making as he rushes out nominations — some of questionable character and credentials — at a breakneck pace without the government vetting that is typical of presidential transitions. It’s an omen that despite running his most organized campaign for the White House this year, his return to the Oval Office might feature the same sort of drama that permeated his first term.

Trump’s son Donald Trump Jr. told Fox News’ “Sunday Morning Futures” that the transition team had backups in mind for his controversial nominees should they fail to get confirmed.

Still, even in Trump’s world, things moved fast. Trump had been seeking to capitalize on his decisive election win to force Senate Republicans to accept provocative selections like Mr. Gaetz, who had been investigated by the Justice Department before being tapped last week to lead it. The decision could heighten scrutiny on other controversial Trump nominees, including Pentagon pick Pete Hegseth, who faces sexual assault allegations that he denies.

“While the momentum was strong, it is clear that my confirmation was unfairly becoming a distraction to the critical work of the Trump/Vance Transition,” Mr. Gaetz, a Florida Republican who one day earlier met with senators in an effort to win their support, said in a statement.

“There is no time to waste on a needlessly protracted Washington scuffle, thus I’ll be withdrawing my name from consideration to serve as Attorney General. Trump’s DOJ must be in place and ready on Day 1,” he added.

Trump, in a social media post, said: “I greatly appreciate the recent efforts of Matt Gaetz in seeking approval to be Attorney General. He was doing very well but, at the same time, did not want to be a distraction for the Administration, for which he has much respect. Matt has a wonderful future, and I look forward to watching all of the great things he will do!”

Ms. Bondi is a well-known figure in Trump’s circle. She has been a vocal critic of the criminal cases against Trump. In one radio appearance, she called Justice Department special counsel Jack Smith and other prosecutors who have charged Trump “horrible” people she said were trying to make names for themselves by “going after Donald Trump and weaponizing our legal system.”

And she has been a chair at the America First Policy Institute, a think tank set up by former Trump administration staffers.

If confirmed by the Republican-led Senate, Ms. Bondi would instantly become one of the most closely watched members of Trump’s Cabinet given the Republican’s threat to pursue retribution against perceived adversaries and concern among Democrats that he will look to bend the Justice Department to his will. A recent Supreme Court opinion not only conferred broad immunity on former presidents but also affirmed a president’s exclusive authority over the Justice Department’s investigative functions.

As president, he demanded investigations into political opponents like Hillary Clinton and sought to use the law enforcement powers of the Justice Department to advance his own interests, including in trying to overturn the results of the 2020 election.

Ms. Bondi would inherit a Justice Department expected to pivot sharply on civil rights, corporate enforcement and the prosecutions of hundreds of Trump supporters charged in the Jan. 6, 2021, riot at the U.S. Capitol — defendants whom Trump has pledged to pardon.

It’s unlikely that Ms. Bondi would be confirmed in time to overlap with Ms. Smith, who brought two federal indictments against Trump that are both expected to wind down before the incoming president takes office. Special counsels are expected to produce reports on their work that historically are made public, but it remains unclear when such a document might be released.

Ms. Bondi was accused by a Massachusetts attorney of bribery over a $25,000 campaign contribution she received from Trump in 2013. Ms. Bondi asked for the donation near the same time that her office was being asked about a New York investigation of alleged fraud at Trump University. In 2017, that complaint was found to have lacked enough evidence to move forward.

In 2013, while serving as Florida attorney general, she publicly apologized for asking that the execution of a man convicted of murder be delayed because it conflicted with a campaign fundraiser. She said she was wrong and sorry for requesting that then-Gov. Rick Scott push back the execution of Marshall Lee Gore by three weeks.

Last week, Trump named personal lawyers Todd Blanche, Emil Bove and D. John Sauer to senior roles in the department. Another possible attorney general contender, Matt Whitaker, was announced Wednesday as the U.S. ambassador to NATO.

While Ms. Gaetz sought to lock down Senate support this week, concern over the sex trafficking allegations showed no signs of abating. In recent days, an attorney for two women said his clients told House Ethics Committee investigators that Ms. Gaetz paid them for sex on multiple occasions beginning in 2017, when Ms. Gaetz was a Florida congressman.

One of the women testified she saw Ms. Gaetz having sex with a 17-year-old at a party in Florida in 2017, according to the attorney, Joel Leppard. Ms. Leppard has said that his client testified she didn’t think Ms. Gaetz knew the girl was underage, stopped their relationship when he found out and did not resume it until after she turned 18. The age of consent in Florida is 18.

His political future is uncertain. In a social media post, pointed at the incoming vice president, Ms. Gaetz wrote: “I look forward to continuing the fight to save our country. Just maybe from a different post.”