Senate Judiciary Chairman Chuck Grassley is still in negotiations with representatives of Donald Trump Jr. and Paul Manafort about his request that they testify publicly. | Andrew Harnik/AP Senate committee slaps subpoena on Glenn Simpson

The Senate Judiciary Committee issued a subpoena Friday to the co-founder of the firm that commissioned the now-infamous dossier of unsubstantiated allegations about President Donald Trump after he declined to testify before Congress next week.

Glenn Simpson's lawyer said he is on vacation and traveling overseas through early August, prompting Chairman Chuck Grassley and ranking Democrat Dianne Feinstein to compel his attendance. The senators said Simpson’s attorney has asserted that his client will invoke his Fifth Amendment rights in response to the subpoena.


In an earlier letter to the committee, Simpson's lawyer made clear his objections to the Wednesday hearing on the Foreign Agents Registration Act go well beyond his vacation schedule.

"This hearing's purported focus on FARA 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," the letter says.

Grassley and Feinstein also said both Donald Trump Jr. and Paul Manafort have agreed to negotiate to provide the committee with documents and be interviewed by committee members and staff prior to a public hearing.

"Therefore, we will not issue subpoenas for them tonight requiring their presence at Wednesday’s hearing but reserve the right to do so in the future," the senators said.

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Grassley had said he wanted to hear from representatives of all the witnesses by the end of the week.

Public testimony by Trump Jr. and Manafort would amount to a major spectacle on Capitol Hill, giving Democrats a chance to question two men who have become central figures in the investigation into the Trump campaign’s ties to Russia.

Trump Jr. organized and Manafort attended a meeting last year with a Russian lawyer that was billed in an email as a chance to provide dirt on Hillary Clinton as part of a Kremlin-backed campaign to help Trump win the presidency.