The founder of the Washington political opposition-research firm that commissioned the anti-Donald Trump dossier will testify next Wednesday before the Senate Judiciary Committee.

Glenn Simpson, one of the Russian election-meddling saga’s most mysterious and sought-after characters, will answers questions from a committee that has pressed for his appearance since early this spring.

In 2015, Mr. Simpson’s firm, Fusion GPS, hired former British intelligence agent commissioned Christopher Steele to compile opposition research on then-candidate Trump. For months Mr. Steele’s salacious 35-page dossier of unverified information was cited by Democrats as the reason for a special commission to investigate Mr. Trump and his aides for a supposed role in Russia’s hacking of Democratic Party email servers.

Three men — Mr. Trump’s attorney, a campaign volunteer and a tech company CEO — have publicly said the parts about them in the dossier are fiction. A Russian diplomat whom Mr. Steele accused of wrongdoing also called the dossier a fantasy.

Mr. Simpson, who founded Fusion GPS in 2009, worked previously for Roll Call newspaper and the Wall Street Journal.

Iowa Sen. Charles E. Grassley, chairman of the Judiciary committee, was the driving force behind securing Mr. Simpson’s testimony. Thus far, Fusion GPS has refused to disclose the identity of political clients who financed the anti-Trump dossier — a line of questioning likely to cause major sparks to fly next week.

On Wednesday, the Senate Judiciary Committee, which is scheduled to explore enforcement of the Foreign Agents Registration Act, will also hear testimony from British-American businessman Bill Browder.

Mr. Browder has called Mr. Simpson a “professional smear campaigner.”

Before the Trump dossier, Fusion GP worked on efforts to repeal the Magnitsky Act, a law enacted by the Obama administration in 2012 to punish Russian officials responsible for the death of Russian lawyer and whistleblower Sergei Magnitsky in a Moscow prison in 2009.

Magnitsky served as an attorney for Mr. Browder.

Mr. Grassley also said this week that he wants to hear testimony from former Trump campaign chairman Paul Manafort, who is not currently on Wednesday’s witness list.

Mr. Grassley told CNN Thursday the committee has also requested Donald Trump Jr. testify before the committee.

News in Washington this week was dominated by revelations that the younger Mr. Trump met with a Russian lawyer while his father campaigned for the presidency. He said he met with the lawyer, Natalia Veselnitskaya, because he thought she had negative information to divulge about his father’s opponent, Hillary Clinton.

Mr. Trump said that instead of discussing Ms. Clinton, Ms. Veselnitskaya instead discussed the Magnitsky Act.

It is not publicly known if Ms. Veselnitskaya had any interactions with Fusion GPS.

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