The co-founder of the Washington-based firm that commissioned the explosive and largely unsubstantiated anti-Trump campaign research dossier will reject a Senate subpoena to testify before Congress next week and invoke his Fifth Amendment rights, according to the heads of the Senate Judiciary Committee.

Glenn Simpson, the former Wall Street Journal reporter who helped start political intelligence firm Fusion GPS, has been a key figure in the Russian election meddling saga ever since the dossier, which alleged a years-long Kremlin conspiracy to elect Donald Trump and included colorful sex stories, was leaked to the press after the Republican’s November victory over Democrat Hillary Clinton.

Since March, the Senate Judiciary Committee has pressed for Mr. Simpson’s testimony and documents relevant to the case. His attorneys, Joshua A. Levy and Robert F. Muse, have rebutted with numerous defenses, including citing confidentiality agreements between Fusion GPS its clients. This week, a Judiciary hearing which would have featured Mr. Simpson — had he voluntarily attended — was canceled.

Late on Friday committee leadership executed hardball tactics to force him to tell what he knows as chairman Sen. Chuck Grassley, an Iowa Republican, and ranking member Sen. Dianne Feinstein, a California Democrat, announced they’d subpoenaed him.

“Glenn Simpson, through his attorney, has declined to voluntarily attend Wednesday’s Judiciary Committee hearing regarding compliance with the Foreign Agents Registration Act,” the senators wrote in a statement. “Therefore, a subpoena has been issued to compel his attendance. Simpson’s attorney has asserted that his client will invoke his Fifth Amendment rights in response to the subpoena.”

Mr. Simpson’s lawyers in a letter reported by Politico, argued that “this hearing’s purported focus on FARA [Foreign Agents Registration Act] is pretext for an exploration of Fusion GPS’ reported work, on behalf of other clients, to investigate the ties of Donald J. Trump, his campaign and their associates to Russia.”

Wednesday’s committee hearing is also scheduled to feature Donald Trump Jr. and President Donald Trump’s one-time campaign manager Paul Manafort.

On Friday, Mr. Grassley and Ms. Feinstein said both Trump confidantes had agreed to negotiate to be interviewed and provide relevant documents but the Senators also reserved the right to subpoena them if necessary.

Legal battle to continue

More legal battle is expected between Mr. Simpson and the committee.

Washington insiders woke Saturday debating whether the political operative’s strategy to assert his Fifth Amendment rights — to protect himself against self-incrimination —- would hold up — or if he’d put himself at risk of being held in contempt of Congress, which could mean future criminal charges.

Deeper digging into the dossier could also be embarrassing for both Democrats and Republicans.

Mr. Simpson hired former British intelligence agent Christopher Steele in 20015 to compile opposition research on then-candidate Trump. The resulting dossier — which was reportedly sourced from the Kremlin — allegedly received initial financial support from anti-Trump Republicans before being taken over and distributed by Democrats. It contained a lurid and largely discredited tale of a years-long Russian effort to elect the former reality TV star and property developer.

The Trump White House has vigorously denounced the allegations as a “pile of garbage” and “FAKE NEWS!” ever since online news service BuzzFeed posted all 35 pages.

This week The Washington Times reported that the FBI is routinely using the dossier as a checklist that agents tick off as they question witnesses in its Russia investigation. Sources told The Times it was strange that a gossip-filled series of memos is guiding the bureau’s work.

For months Mr. Grassley has pushed to learn more about Mr. Steele’s FBI relationships, which allegedly date back to help the former British spy provided the bureau’s investigation of FIFA chief Sepp Blatter, whose 17-years reign over the football World Cup governing body ended amid corruption allegations.

Former FBI Director James Comey, whom Mr. Trump fired in May, has refused to publicly answer questions about the bureau’s relationship with Mr. Steele.

Additional drama next week is expected from Mr. Trump Jr. and Manafort’s testimony. Democrats are eager to question both about a meeting they held last year with a Russian lawyer who promised to provide comprising information about Ms. Clinton. Republicans are keen to start clearing the air on an issue that has distracted Washington from Mr. Trump’s agenda.

On Friday, Reuters reported it had found public records contradicting statements by the lawyer who met Mr. Trump Jr. and Mr. Manafort that she’d never worked for the Russian government. The news agency found Natalia V. Veselnitskaya had once represented Russia’s top intelligence agency, the F.S.B., which replaced the K.G.B. after the fall of the Soviet Union.

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