Senate Republicans are demanding the Justice Department and FBI retain and preserve all records related to special counsel Jack Smith’s prosecutions of President-elect Donald Trump.

In a letter sent this week to Attorney General Merrick Garland, FBI Director Christopher A. Wray and Mr. Smith, Sens. Charles E. Grassley, Iowa Republican, and Ron Johnson, Wisconsin Republican, demanded the departments protect documents related to their probes of Mr. Trump for congressional oversight purposes.

The two lawmakers say that according to whistleblowers, former FBI agent Timothy Thibault was involved in the decision to open up probes into Mr. Trump that Mr. Smith later prosecuted. 

According to the senators, Mr. Thibault was found to have violated the Hatch Act for his anti-Trump partisanship. The Hatch Act prohibits government employees from politicking while on the job.

“Due to the apparent political bias of FBI officials that were involved in the genesis of a case against former President Trump, preservation of special counsel Smith’s records is more important than ever. If a politically charged case is to be opened, it must be done the right way and free from political bias,” Mr. Grassley and Mr. Johnson said in their letter dated Tuesday.

The senators are also questioning whether investigators leaked information to the media.

Their letter noted that the agencies have destroyed evidence in the past that Congress wanted to review. The evidence involved members of special counsel Robert Mueller’s team wiping their phones, plus laptops destroyed in relation to the 2016 probe of Hillary Clinton.

House Judiciary Committee Chairman Jim Jordan, Ohio Republican, and Rep. Barry Loudermilk, Georgia Republican, last week made the same request for Mr. Smith to maintain his records. 

They demanded answers regarding the 2022 search warrant issued on Mr. Trump’s home in the classified documents case.

Mr. Smith has signaled to an appeals court he’s pausing the documents legal battle that he had brought to the 11th U.S. Circuit Court of Appeals after it was dismissed by Judge Alieen Cannon of the Southern District of Florida in July for issues related to Mr. Smith’s standing to bring the case since he was a private citizen when he was made special counsel. 

And the federal judge in Washington has canceled all deadlines related to Mr. Smith’s prosecution of Mr. Trump over the 2020 election.

Mr. Smith plans to resign before Mr. Trump takes office, according to multiple reports.

• This story is based in part on Associated Press reporting.