Feinstein, released the transcript because she said she felt the public had a right to know what the witness said, especially in light of recent claims by Republicans that the Michael Steele-Fusion GPS dossier was prepared to discredit the Trump campaign.
The Washington research firm has been under attack by the White House and Republican lawmakers over the dossier, which is central to investigations in Congress and by a federal special counsel into allegations that Russian Federation interfered in the 2016 presidential election to help Trump win.
The publication of a controversial dossier about President Donald Trump's alleged links with Russian Federation, authored by a former MI6 spy, has already resulted in a death, it has been claimed.
Democratic Senator Dianne Feinstein released a transcript of the interview on Tuesday, after repeated disagreements with Republicans on the committee.
"My understanding was that they believed Chris at this point - that they believed Chris 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", Simpson said.
But in the late stages of the 2016 presidential campaign, Steele broke off with the FBI, Simpson said, because he did not believe it was taking him seriously enough.
Feinstein's decision came after a tense back-and-forth between Republicans and Democrats over what to do with the transcript of Simpson's interview, which he provided on August 22.More news: Job creation is at eight year low, says Rahul Gandhi in Bahrain
In mid-to-late September 2016, Steele told Simpson that the Federal Bureau of Investigation had asked him to provide more information, Simpson testified.
Feinstein said she released the transcript to set the record straight.
In their letter, Whitehouse and Blumenthal echoed complaints by Fusion GPS that Simpson's testimony has been selectively leaked, and they rejected the idea the company's work was the basis for the start of the investigation into Russia's interference with the 2016 election. He has also talked to the House and Senate intelligence committees.
Both Hillary Clinton's campaign and the DNC hired Fusion GPS for its opposition research services during the 2016 campaign.
The 35-page report drew on information from Russian contacts and concluded that Russia had been "cultivating, supporting and assisting" Trump for at least five years and fed his campaign "valuable intelligence" about Clinton.
Virginia Sen. Mark Warner, the top Democrat on the Senate Intelligence Committee, which has also interviewed members of Fusion GPS, also appeared to back Grassley's rationale last week. Lindsay Graham last week asked the Justice Department to investigate Steele, saying they had information that he may have made false statements to the government.
Mr Simpson declined to say who he suspected the whistle-blower to be, but did not think it was a source of Mr Steele's.