Corey Lewandowksi, Donald Trump’s 2016 campaign manager and a key figure in his political orbit, is set to appear before the House Judiciary Committee where he will be questioned in a public hearing on Sept. 17, according to two sources familiar with the discussions.

Lawmakers are interested in pressing Lewandowski for more information on the instances of possible obstruction of justice by the president that were outlined in Robert Mueller’s report. Though Lewandowski did not hold a White House job, he figures prominently in Volume 2 of the report, which found that Trump asked his former campaign chief to press then-Attorney General Jeff Sessions to curb the special counsel’s investigation.

In August, Judiciary Chairman Jerry Nadler (D-N.Y.) issued a subpoena for testimony from Lewandowski, who has publicly said he’d be happy to testify and, as he put it during a Fox News radio interview, “remind the American people that these guys are on a witch hunt, right?"

Lewandowski’s appearance before the committee represents a much-needed win for Democrats, who are struggling to haul key fact witnesses in the Mueller report to the Hill for further questioning. It would be the first appearance of such a fact witness since former White House communications director Hope Hicks testified behind closed doors in June. She repeatedly invoked claims of executive privilege to decline offering answers, and Democrats expect Lewandowski to be an even more evasive and combative witness.

Unlike Hicks, however, Lewandowski will answer questions in an open hearing with the cameras rolling—making him the first Trump associate to do so before Nadler’s committee. The spectacle may inject some life into Democrats’ impeachment investigation, which Nadler and other leaders have made far more explicit over the summer ahead of a crucial crossroads this fall.

The committee also served subpoenas for two other Trump associates—deputy White House chief of staff Rick Dearborn and former White House staff secretary Rob Porter—to appear on Sept. 17, too. No testimony from either of those former officials has been confirmed.

The high-profile testimony will also come amid speculation that Lewandowski may mount a run for U.S. Senate in his home state of New Hampshire, which he has teased heavily in recent months.