Two Republican chairmen of powerful House committees have asked Attorney General Jeff Sessions and Deputy Attorney General Rod Rosenstein to appoint a special counsel to investigate “potential bias” within the FBI in 2016 and 2017 as the bureau obtained surveillance warrants related to the investigation into Russian interference in the 2016 U.S. election.

“The public interest requires the appointment of a Special Counsel” to investigate “certain decisions made and not made by the Department of Justice and FBI in 2016 and 2017” due to potential political conflicts of interest, Reps. Trey Gowdy of South Carolina and Bob Goodlatte of Virginia wrote in a letter to Sessions and Rosenstein on Tuesday.

Gowdy chairs the House Oversight and Government Reform Committee and Goodlatte is chairman of the House Judiciary Committee.

The request appears to stem from allegations contained in a memo released by House Intelligence Committee chairman Devin Nunes. The memo alleges FBI investigators relied on unsubstantiated evidence from a dossier compiled by Democratic opposition researchers to obtain surveillance warrants through Foreign Intelligence Surveillance Act against members of President Donald Trump’s campaign team.

Watch: Intelligence Officials Aware of Russian Activity Aimed at 2018 Elections

“There is evidence of bias, trending toward animus, among those charged with investigating serious cases. There is evidence political opposition research was used in court filings.  There is evidence this political opposition research was neither vetted before it was used nor fully revealed to the relevant tribunal,” Gowdy and Goodlatte wrote in the letter.

Sessions last month instructed DOJ Inspector General Michael Horowitz to investigate potential FISA abuses. IG investigations cannot compel key witnesses to testify.

The president lashed out at Sessions on Twitter for not more rigorously pursuing potential FISA abuses.

“Will take forever, has no prosecutorial power and already late with reports on Comey etc.,” Trump tweeted about an inspector general investigation. 

House Democrats have dismissed Gowdy and Goodlatte’s letter as partisan nonsense issued primarily to temper the president’s indignation over the ongoing Russia investigation that has plagued his presidency.

“As if on cue, House Republicans now call for the appointment of a special counsel to investigate ‘FISA abuse,’” Rep. Adam Schiff of California, ranking member of the House Intelligence Committee, said in a statement Tuesday. “In so doing, Republicans express their interest in investigating everything but what is most important: Russia’s interference in our election, the Trump campaign’s role and what we need to do to protect the nation in the future. This may help serve the President’s interests, but does nothing to serve the national interest.”

For the Department of Justice to establish a special counsel, it must determine that a “criminal investigation of a person or matter is warranted,” according to federal regulation.

“In this case, it is difficult to see what crime the Republicans are alleging,” Rep. Elijah Cummings of Maryland, ranking member of the Oversight Committee, said in a statement Tuesday. “Republicans have made numerous unsubstantiated allegations, like claiming that the Justice Department did not disclose in its FISA application that research was funded by political opponents.  But even if that claim were true, it does not constitute criminal activity.”

Gowdy indicated Tuesday that Republicans would not launch investigations into the alleged bias and FISA abuses at the DOJ because such investigations lacked public confidence and certain prosecutorial tools.

Parts of the executive branch are “more publicly confidence-inspiring than current congressional investigations,” he told CBS News.