Matt Gaetz, under House ethics investigation, is Trump's pick for attorney general

President-elect Donald Trump announced on Wednesday that he has chosen Rep. Matt Gaetz as his pick for attorney general, a move that, if he's confirmed by the Senate, would place a firebrand and one of Trump's most loyal allies at the head of the Justice Department.

"Matt is a deeply gifted and tenacious attorney, trained at the William & Mary College of Law, who has distinguished himself in Congress through his focus on achieving desperately needed reform at the Department of Justice," Trump said in his social media post.

Gaetz is an explosive selection by Trump to be the chief law enforcement officer of the federal government, leading the very same executive branch of government that spent years investigating allegations regarding the Florida congressman. Gaetz was informed that the Justice Department would not seek changes just last year. He has long denied any wrongdoing.

House Speaker Mike Johnson announced later Wednesday that Gaetz offered his resignation from Congress effective immediately.

Johnson said the resignation took him by "surprise" but that the Florida congressman did so to "start the clock" on the process for Florida Gov. Ron DeSantis to call a special election to fill the vacancy.

"So Matt would have done us a great service by making that decision, as he did, on a fly. And so we're grateful for that so we move forward," Johnson added.

Gaetz's resignation will narrow the slim majority that Republicans will have in the new Congress in January, but he represents a reliably Republican district in the Florida Panhandle.

Gaetz has been a member of Congress since winning in 2017, riding the MAGA wave that brought Trump to Washington eight years ago. Over the years, Gaetz has become one of Trump's most ardent, and according to some allies, effective, defenders in Washington while also growing close to Trump.

Gaetz has been down at Trump's residence in Mar-a-Lago almost every day since Election Day, helping make suggestions and input on other administration selections, sources tell ABC News. Gaetz was also seen traveling with Trump in his motorcade during his visit to Washington on Wednesday.

Notably, Gaetz is very close with Trump's newly selected chief of staff, Susie Wiles, who also has deep and storied roots in Florida politics.

Beginning in 2019, Gaetz faced a yearslong Justice Department investigation into allegations related to sex trafficking and obstruction of justice. Gaetz has long denied any wrongdoing, and the Justice Department informed Gaetz in 2023 that it was declining to bring charges against him after its investigation.

The investigation into Gaetz stemmed from a probe into the Florida congressman's one-time friend, former Seminole County Tax Collector Joel Greenberg, who was sentenced in 2022 to 11 years in federal prison after pleading guilty to multiple charges, including sex trafficking a minor and introducing the minor to other "adult men."

Since the Justice Department declined to charge Gaetz following its investigation, the Florida congressman has faced an ongoing probe by the House Ethics Committee regarding the same allegations.

In September, Gaetz released a lengthy statement concerning the ongoing House Ethics probe into his alleged conduct. Gaetz stated that he would no longer voluntarily participate in the probe and included a string of answers seemingly to questions the committee asked the Florida congressman earlier that month.

The House Ethics Committee drops an investigation into a member once they leave Congress, House Ethics Committee Chairman Michael Guest told ABC News.

"You know, once a member is no longer a member of Congress, then ethics has no jurisdiction. So if Matt Gaetz were to be appointed as the Attorney General, the ethics investigation is currently ongoing, would cease at that point," Guest said.

House Republicans were meeting behind closed doors when Trump announced he would be nominating Gaetz to serve as attorney general. There was an audible gasp in the room, several members who were inside told ABC News.

Many House Republicans on Capitol Hill are reacting to Trump's pick of Gaetz.

House Majority Leader Steve Scalise and Rep. Guest expressed support for Trump's selection of Gaetz as attorney general. Republican Rep. Max Miller, a former aide to Donald Trump who was particularly critical of Gaetz following Florida congressman's efforts to oust former Speaker Kevin McCarthy last year, said the pick was "silly" and that Gaetz would "never get confirmed by the Senate."

"I believe that the President is probably rewarding him for being such a loyal soldier to the president, but the President is smart enough, and his team is smart enough to know that Mr. Gaetz will never get confirmed by the Senate. Whatsoever," Miller said.

ABC News' Rachel Scott, Lauren Peller and Isabella Murray contributed to this report.