Feinstein also sits on the Senate intelligence committee, which is conducting its own investigation into the Russian interference and whether Trump's campaign was in any way involved.
Democrats on the committee said they were not consulted ahead of a decision by Senate Republicans to issue a criminal referral against former British spy and Fusion GPS investigator Christopher Steele.
Through much of 2017, Feinstein and Grassley made joint requests for information about Russian Federation and the FBI's investigation of election interference. According to Simpson, Chris thought the DNC hack was a serious national security matter and Fusion was obligated to tell someone in the US government.
"The Fusion GPS chief's lawyer, Joshua Levy, interrupted to say: "[Mr Simpson] wants to be very careful to protect his sources.
Senator Grassley, who at almost every turn has worked to protect President Trump, on Tuesday blasted Senator Dianne Feinstein (D-CA), who released the bombshell transcript just a few hours earlier.
"Her action undermines the integrity of the committee's oversight work and jeopardizes its ability to secure candid voluntary testimony relating to the independent recollections of future witnesses", Grassley's spokesman, Taylor Foy said in a statement.
The split between Feinstein and Grassley has widened in recent weeks. That detail is one of many in a transcript released today by Senator Dianne Feinstein.
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More dramatically, Grassley and Sen.
She told an NBC News reporter that she regrets not speaking to Grassley beforehand, citing her cold as a possible factor.
In urging the committee to release the full transcript of his interview, Simpson has argued that Republicans are trying to obscure what happened in 2016.
Steele authored a dossier that revealed several alarming issues about Trump, including his questionable business dealings within Russian Federation as well as an alleged incident involving the candidate previously hiring prostitutes to urinate on a bed once slept in by former President Barack Obama. Two months later, at the FBI's request, Steele gave a more in-depth briefing to bureau officials at a meeting in Rome.
Simpson elaborated on this in the interview and said the Federal Bureau of Investigation "believed Chris's information might be credible because they had other intelligence that indicated the same thing and one of those pieces of intelligence was a human source from inside the Trump organization".
Steele "severed his relationship" with the FBI after the Times reported the bureau found no Trump ties to Russian Federation.
Simpson touched on a wide array of topics in the 312-page transcript, including President Trump's business ties and his tax history.