The co-founder of the opposition-research firm that commissioned a Donald Trump dossier is set to testify next Wednesday before the Senate Judiciary Committee.

Glenn Simpson, co-founder of Fusion GPS, is set to appear as one of several witnesses at a hearing on enforcement of the Foreign Agents Registration Act, the committee announced Wednesday.

Simpson’s firm in 2015 commissioned Christopher Steele, a former British intelligence agent, to dig up opposition research on then-candidate Trump.

Steele’s salacious and largely unsubstantiated findings were later leaked to news outlets and published by BuzzFeed.

The dossier alleges Trump and his campaign colluded with the Russian government to damage Hillary Clinton.

Earlier Wednesday, Senate Judiciary Chairman Chuck Grassley (R-Iowa) indicated that he wanted former Trump campaign chairman Paul Manafort to appear at next week’s hearing, but Manafort is not on the witness list.