Democrats have also criticized a memo Barr wrote to the Justice Department before his nomination in which he criticized Mueller's investigation for the way it was presumably looking into whether Trump had obstructed justice.
Barr previously was in several high-level DOJ positions, including as US attorney general in the George H.W. Bush administration. His nomination will be headed to the Senate floor for a full vote.
Nadler said he might need the "threat" of the subpoena to get answers during the Friday session.
Barr has declined to offer a firm guarantee that he would release Mueller's report in its entirety - noting that the regulations governing Mueller's appointment do not call for that.
The GOP head of the Senate Intelligence Committee says after two years of investigation, the panel still has not found evidence of collusion between the Trump campaign and Russian Federation. Senate Democrats expressed concern that Barr would not make the results of the investigation public. Democrats have also criticized Barr for not committing to publicly releasing Mueller's findings.
A year ago he sent an unsolicited legal opinion to the Justice Department and White House arguing that Mueller had no grounds to investigate Trump for obstruction of justice based on the firing of Federal Bureau of Investigation director James Comey in May 2017.
Whitaker, who was appointed by Trump in November, said last week that the investigation "is close to being completed".
"I have concluded that Mr. Barr is qualified for the position & his record strongly suggests he will exercise independent judgment & uphold the best interests of the DoJ". That, combined with Barr's sweeping view of executive power, prompted Leahy to vote no on Barr's nomination. "I don't know what Bob Mueller is writing", Barr said at his hearing last month.
The Republican president has repeatedly criticized the investigation as a "witch hunt" and denies any collusion with Moscow.