The subpoena from the Republican-led Senate Intelligence panel is believed to be the first issued to one of Trump's family members.
If confirmed, it would be first time one of President Trump's children is facing a congressional subpoena. "Throughout the investigation, both senators have said publicly that the committee would need to re-interview some witnesses", they said in a joint statement. Trump Jr. declined to speak with the special counsel's investigators, and they did not subpoena him.
Trump Jr.'s position on testifying again hardened after the Mueller report was released, according to one of the sources.
Most Republicans this week have rallied behind the president's effort to quash the lingering investigations raised by Mueller's report, with Senate Majority Leader Mitch McConnell, R-Kentucky, on Tuesday declaring, "Case closed". Richard Burr of North Carolina, at one point shared details about the FBI's Russian Federation investigation with the White House counsel's office.
Donald Trump Jr is expected to fight the subpoena, according to CNBC referring to a source.
"[Donald Trump Jr.] has already spent dozens of hours testifying in front of Congressional committees".
The committee is one of the few in the Senate that has been conducting serious investigations related to Russian meddling in the 2016 US presidential election. Trump Jr. told the Senate Judiciary Committee in 2017 he was only "peripherally aware" of the proposal.
The 448-page Mueller report summed up the results of the special counsel's 22-month long investigation into allegations that Trump's team conspired with Moscow to influence the outcome of the 2016 election and that the United States president had obstructed justice.
He testified before the committee in 2017 about his participation in a June 2016 meeting at Trump Tower with Russians offering "dirt" on Hillary Clinton.
Warner said that the intelligence panel also wants to see the underlying evidence in Mueller's report, a request that House lawmakers have also made.
Mulvaney, in an interview with CBS News, downplayed the significance of the subpoena for the younger Trump since he does not work in the White House.
Mueller's report says that Cohen testified he had discussed the project on multiple occasions with Trump Jr.
The proposed tower in Moscow was the brainchild of Cohen and Felix Sater, a former Trump associate who chased property deals for the future president.