Senate Minority Leader Charles E. Schumer asked Tuesday for the Justice Department inspector general to probe President Trump’s past communications with Matthew G. Whitaker, the man he designated to be acting attorney general.

Mr. Schumer said he sees a pattern of “unusual contacts” between Mr. Trump and Mr. Whitaker dating back to last year. The New York senator suspects the president may have been testing the waters to see if Mr. Whitaker would foil the ongoing special counsel’s investigation into the 2016 elections, Russian meddling and Trump campaign figures’ behavior.

Mr. Schumer specifically said he wants the inspector general to look at whether Mr. Whitaker shared confidential grand jury information with the White House.

“The Department of Justice must exercise its prosecutorial powers free from partisan considerations, and special counsel Robert Mueller must be allowed to conduct his investigation free from political interference and unlawful or unauthorized disclosures,” Mr. Schumer wrote in a letter to Inspector General Michael Horowitz. “Protecting this independence is essential to preserving the rule of law and justice in the United States.”

If Mr. Horowitz takes up the investigation, it would be yet another politically charged probe on his list. Earlier this year, he concluded an investigation into the Justice Department’s handling of the Clinton emails, in a probe that exposed the embarrassing text messages between then-FBI Agent Peter Strzok and then-FBI lawyer Lisa Page.

He’s also looking into the initial handling of the Russia investigation and decisions to conduct some forms of snooping on Trump campaign figures.

Mr. Schumer’s request, meanwhile, is the latest salvo from congressional Democrats who argued Mr. Trump’s presidency was clouded by potential collusion with Russia.

While no firm evidence of collusion has emerged yet, they say the Mueller probe’s success in winning guilty pleas and convictions of Trump campaign figures on other charges such as lying to the FBI or financial shenanigans predating the campaign means the probe is going after live targets.

Democrats fear Mr. Whitaker will constrain the special counsel’s 18-month-old investigation.

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