Senate Judiciary Committee members seeking to to shine light into one of the sleaziest, darkest corner of the Russian election meddling saga will have to wait a little longer.

On Wednesday, they were scheduled to question the figure who supported the anti-Donald Trump dossier, Washington political operative Glenn Simpson, but committee Chairman Sen. Charles E. Grassley, Republican of Iowa, late Tuesday postponed the hearing. Mr. Grassley’s office offered no explanation for the delay but sources told The Washington Times that Mr. Simpson never intended to comply with the request to appear, which was voluntary.

Mr. Simpson’s name cropped up earlier this month amid news that Donald Trump Jr. met with a Russian lawyer during the presidential campaign after being promised information helpful to his father’s effort. On Wednesday it emerged that Mr. Trump Jr.’s meeting with Natalia Veselnitskaya also included a representative of the Russian developer who partnered with his father to bring the Miss Universe pageant to Moscow. The Russian developer is also suspected of money laundering.

Mr. Simpson has been a central figure in the Russia story since it emerged early this year that he hired former British intelligence agent Christopher Steele in 2015 to compile opposition research on then-candidate Trump. The dossier shook Washington with a lurid and largely discredited tale of a years-long Kremlin conspiracy to elect the former reality TV star and property developer, in addition to colorful sex stories.

Mr. Trump vigorously denounced the dossier as a “pile of garbage” and on Jan. 13 tweeted that it was: “Totally made up facts by sleazebag political operatives, both Democrats and Republicans — FAKE NEWS! Russia says nothing exists. Probably …”

Three men named in it — Mr. Trump’s attorney Michael Cohen, campaign volunteer Carter Page and tech company CEO Aleksej Gubarev — have also called it fiction.

For months Mr. Grassley has pressed for a more methodical inquiry into the 35-page document, which privately circulated among a handful of Washington’s top-shelf politicians, intelligence chiefs and journalists before being published by BuzzFeed.

Of the multiple congressional probes into Russia, the Senate Judiciary Committee has stood out for its tight focus on the anti-Trump dossier. The panel has asked Mr. Simpson’s firm, Fusion GPS, to provide more details on the ex-spy Mr. Steele, and documents or answers to multiple questions, including who paid for the dossier.

Committee investigators particularly want to learn more about Mr. Steele’s reported relationship with the FBI and his role in helping the bureau investigate FIFA chief Sepp Blatter — whose 17-years reign over the football World Cup governing body ended amid corruption allegations.

For months Mr. Simpson and his lawyer, Joshua A. Levy, have battled back against Mr. Grassley citing numerous defenses, including their First Amendment rights, attorney client privilege, attorney work product and confidentiality agreements between Fusion GPS its clients.

The committee has countered that those claims were invalid because Fusion GPS appeared to share some version of the dossier with “journalists, members of Congress, and the FBI.”

When the dossier first emerged, it triggered a surge of questions about last year’s presidential campaign, including what exactly did the Russians do to influence the November election, what did Trump campaign officials and Obama officials know and who leaked what.

In May, Mr. Trump became only the second president to fire his FBI director, James B. Comey, citing the agency’s probe in the Russia issue as a reason. Mr. Comey also briefed Mr. Trump on the existence of the dossier.

On Wednesday, senators were also slated to hear from an anti-Kremlin American businessman central to Russia’s animosity toward Washington, Bill Browder. Mr. Browder was the driving force behind the Magnitsky Act, which was passed by Congress in 2012 in response to the dubious death of a Russian attorney named Sergei Magnitsky — who was Mr. Browder’s lawyer.

Mr. Magnitsky died in prison after exposing a tax fraud scheme and the law that bears his name has allowed the U.S. to impose sanctions on Russians deemed human rights violators.

Capitol Hill was braced for Wednesday’s drama because Mr. Browder’s work on behalf of Magnitsky was in opposition to Mr. Simpson — whose firm Fusion GPS was involved in an unsuccessful effort to repeal the act. Mr. Browder has called Mr. Simpson a “professional smear campaigner.”

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