In the wake of special counsel Robert Mueller's testimony to Congress, U.S. Sen. Edward Markey and Rep. Katherine Clark both called Thursday for the start of an impeachment inquiry into President Donald Trump over Russian election interference and the president's possible attempts to obstruct justice.

"The evidence in the Mueller report and the Special Counsel's testimony reaffirming it compel Congress to do what is right and necessary. And that is to exercise our authority and begin an impeachment proceeding against Donald Trump," Markey said in a statement on Twitter. "Nothing less than our democracy is at stake."

Markey has two Democratic primary challengers who have already said they support impeachment.

Clark, who is a vice chair of the House Democratic caucus, said hearings, subpoenas lawsuits and other steps taken by Democrats short of an impeachment inquiry have been met with "stonewalling and obstruction."

"I do not make this decision lightly," Clark said. "An impeachment inquiry is a process, not an outcome, and with a heavy heart, I acknowledge that there is no amount of wrongdoing that we could uncover that would convince Senate Republicans to hold the President accountable. With that said, regardless of the outcome, I believe we have a patriotic duty to uncover the facts for the American people."

Rep. Lori Trahan of Lowell previously called this week for a formal impeachment inquiry following Mueller's testimony. In the Massachusetts delegation, Reps. Richard Neal, Stephen Lynch and William Keating still have not said they would support formal impeachment hearings.