Rep. Michael G. Grimm (R-N.Y.) was defiant in the face of 20 tax and business fraud charges filed against him Monday by prosecutors in Brooklyn, vowing to “fight tooth and nail until we’re exonerated.”

Grimm accused prosecutors of misconduct in the case, citing leaks to the media, and he promised to serve out his term and win reelection this fall. He later told House Republican leaders, however, that he would resign from the powerful Financial Services Committee until the charges were resolved.

Republicans face the prospect that the indictment could cost them their only congressional seat in New York City, as Grimm was already facing a formidable challenge from former City Council member Domenic M. Recchia Jr., a Democrat.

Loretta E. Lynch, the U.S. attorney for the Eastern District of New York, released the 30-page indictment along with a strongly worded attack against the former FBI fraud investigator. “Michael Grimm made the choice from going to upholding the law to breaking the law,” Lynch told reporters.

The charges allege that a restaurant owned by Grimm hid from local and federal tax officials more than $1 million in receipts, while also paying workers hundreds of thousands of dollars off the books to further avoid paying taxes. The prosecutors said Grimm then lied about the scheme while giving a deposition in 2013 in a lawsuit filed by workers.

The indictment included five counts each of mail and wire fraud, three counts related to filing false tax returns, two counts of perjury and one count of obstructing the investigation. Other counts are related to hiring illegal immigrants and declining to provide workers’ compensation insurance.

Lynch indicated that the business fraud charges were a smaller piece of a broader investigation into Grimm’s campaign activities that could lead to further charges. “Obviously there is a larger investigation that is still continuing,” she told reporters.

“As a former FBI agent, Representative Grimm should understand the motto: fidelity, bravery, and integrity,” FBI Assistant Director George Venizelos said in a statement. “Yet he broke our credo at nearly every turn. In this twenty-count indictment, Representative Grimm lived by a new motto: fraud, perjury, and obstruction.”

Venizelos said that once this criminal investigation was completed, he expected FBI officials to begin a formal review of all of Grimm’s casework from his time with the bureau to make sure he handled cases properly.

Grimm turned himself in to the FBI at an undisclosed location Monday morning and was released on $400,000 bail after a five-minute arraignment in the federal courthouse.

The congressman put up his Staten Island home as collateral for posting his bond. He also was ordered by the court to surrender his passport and any firearms, and he cannot travel abroad.

He is expected to appear in court May 19 to enter a plea and to determine whether to seek a speedy trial, which is his right. If he chooses that route, the case would conclude before the November election.

Grimm’s resignation from the Financial Services Committee came as House Minority Leader Nancy Pelosi (D-Calif.) sent House Speaker John A. Boehner (R-Ohio) a letter requesting that he remove him.

“These are very serious charges,” Pelosi wrote.

An hour after his arraignment, Grimm walked a block to a nearby veterans memorial to assert his innocence. He invoked his past as a Marine and asserted that resigning would be a betrayal. “I don’t abandon my post,” he said. “I have a lot more service and leadership to provide this country.”

He suggested he had been targeted by establishment New York legal and political forces that wanted to “assassinate my character and remove me from office.”

At one point he suggested he was still humble. “Pride’s a very dangerous thing,” said the lawmaker, who in January threatened to throw a New York reporter off a congressional balcony when he asked Grimm questions about the investigation.

He took just one question from a reporter, who asked whether the lawmaker was “a crook.”

“No,” Grimm said, and then he departed.

Monday evening, more than five hours after he left his news conference, Grimm did not return to the Capitol for the first House votes of the week.