Rep. Jerry Nadler (D-New York), the House Judiciary Committee chairman, has filed a lawsuit against Don McGahn, the former White House Counsel, to enforce a subpoena for his testimony.

Special Counsel Robert Mueller, who investigated possible collusion between Russia and President Donald Trump, extensively interviewed McGahn. But House Democrats want their own evidence as they are investigating Trump’s possible obstruction of justice, which they may use to file articles of impeachment against the president.   

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The suit says that legislators cannot continue their investigation without McGahn’s testimony. It states, “McGahn is uniquely positioned to explain those events, bring additional misconduct to light, and provide evidence regarding the President’s intent.”

Top Republican on the Committee, Rep. Doug Collins (R-Georgia), says the lawsuit is a bad move because it could cause the committee to lose access to valuable White House evidence.

The White House has instructed McGahn to defy the subpoena, saying that the power of the executive branch protects him from having to testify. So-called ‘executive privilege’ was claimed during the Bush and Obama administrations in certain cases. But House lawyers say that government officials are not immune to having to testify in an investigation.

Nadler issued the subpoena after Mueller’s report was released in April. McGahn declined to comply and did not appear for the public testimony on May 21. 

Nadler has told the public that he expects to conclude his investigation and potentially recommend Impeachment proceedings in late fall.