By David Morgan

WASHINGTON (Reuters) - The incoming Democratic chairman of the U.S. House of Representatives Judiciary Committee intends to investigate President Donald Trump's impact on the integrity of the Justice Department and the FBI, according to a letter sent to top federal law enforcement officials on Tuesday.

Representative Jerrold Nadler, now the panel's top Democrat, gave acting Attorney General Matthew Whitaker and FBI Director Christopher Wray until Dec. 31 to respond to more than 100 letters House Democrats have already sent concerning Trump's behavior toward U.S. law enforcement, including verbal attacks on senior officials.

"I write with growing concern over President Trump's repeated attacks on the integrity of the Department of Justice and the FBI," Nadler told Whitaker and Wray in the Nov. 13 letter that was made public.

"The president's behavior appears to be motivated by an urge to shield himself, his family, and his business interests from the ongoing work of the department and the bureau."

Justice Department officials were not immediately available for comment.

Nadler's letter, surfacing a week after U.S. congressional elections gave Democrats control of the House, is an early sign of the heightened scrutiny that Trump and his administration can expect after the new Congress convenes in January.

Democrats are worried that Trump could undermine a federal investigation into Russia's role in interfering with the 2016 presidential election and any collusion by the Trump campaign. The probe is being led by U.S. Special Counsel Robert Mueller.

Trump set off alarms among Democrats last week by forcing the resignation of Attorney General Jeff Sessions and replacing him with Whitaker, a Trump loyalist who has said the Mueller probe should be scaled back.

"The president has called the special counsel's investigation a 'witch hunt' at least 84 times this year," Nadler wrote.

"He has identified the investigation as a significant threat to his administration, stating that he wants it stopped. He has threatened to fire department officials for failing to 'totally protect him'," Nadler continued.

"These actions are not normal," the letter added. "And they ignore the guidance of the White House Counsel, flout the Constitution and undermine our federal law enforcement agencies."

(Reporting by David Morgan; editing by Kevin Drawbaugh and Bill Berkrot)