Previously, the whistleblower had offered to answer questions under oath and in writing if submitted by the House Intelligence Committee as a whole. On Saturday, the chairman said the impeachment inquiry will not be used to undertake investigations into the Bidens or the 2016 election.
Sen. Lindsey Graham (R-S.C.) doubled down on his defense of President Trump amid the ongoing impeachment inquiry, arguing there's "nothing there" in the call between Trump and Ukrainian leaders to suggest the president did anything wrong.
In addition to Biden and the whistleblower, House Republicans also asked for several other testimonies in an open settling, including from Devon Archer, an American businessman who served with Biden on the board of Burisma Holdings; Nellie Ohr, former contractor for opposition research firm Fusion GPS; and David Jale, Under Secretary of State for Political Affairs.
Among those requested to be witnesses for public hearings as part of the GOP-led effort included former Vice President Joe Biden's son Hunter Biden, Tim Morrison, and Kurt Volker - all key players in the current probe against Mr. Trump.
Schiff said three State Department witnesses will appear in two hearings Wednesday and Friday: the USA ambassador to Ukraine, Bill Taylor, career department official George Kent, and Marie Yovanovitch, the former U.S. ambassador to Ukraine.
"The impeachment inquiry, moreover, has gathered an ever-growing body of evidence-from witnesses and documents, including the President's own words in his July 25 call record-that not only confirms, but far exceeds, the initial information in the whistleblower's complaint", Schiff wrote. Republican leadership has complained that the process is unfair and overly restrictive on their ability to question witnesses.
"I've written the whole process off", he said.
Another Democratic member of the committee, Jim Himes, said Sunday that when people hear from respected officials with first-hand knowledge of the pressure campaign against Ukraine "the president's defense... which is all these people had second-hand knowledge, that's going to crumble".
Mr. Schiff replied that the committee is "evaluating the Minority's witness requests and will give due consideration to witnesses within the scope of the impeachment inquiry, as voted on by the House".
Trump has been critical of Schiff for his "parody" reading of Trump's July phone call during a hearing in September.
Three committees have jointly conducted a number of closed-door hearings and the Democrats have released transcripts from those rivate depositions.