President Trump must not try to “interfere” with an investigation that has led to the indictment of a former aide by firing special counsel Robert Mueller, Senate Minority Leader Chuck Schumer warned Monday.

“The president must not, under any circumstances, interfere with the special counsel’s work in any way,” Schumer, a New York Democrat, said Monday. “If he does so, Congress must respond swiftly, unequivocally, and in a bipartisan way to ensure that the investigation continues.”

Former Trump campaign Paul Manafort surrendered to the FBI on Monday morning, along with business partner Rick Gates, after the Justice Department charged the pair with money laundering and conspiracy against the United States. The indictments are the first to emerge from special counsel Mueller’s investigation into Russian interference in the 2016 elections, although they don’t flow directly from their campaign work.

Mueller, a former FBI director with a reputation for integrity, has taken fire from Republicans skeptical that he can pursue an unbiased investigation. That criticism is rooted, in part, in his long-standing friendship with former FBI Director James Comey, whom Trump fired in part due to frustration over the Russia investigation.

“The federal code could not be clearer — Mueller is compromised by his apparent conflict of interest in being close with James Comey,” Rep. Trent Franks, R-Ariz., said Friday. “The appearance of a conflict is enough to put Mueller in violation of the code.”

Schumer rejected such calls and warned Trump not to act on them. “These reported indictments show that the special counsel’s probe is ongoing in a very serious way,” he said. “The rule of law is paramount in America and the investigation must be allowed to proceed unimpeded.”