Sen. Richard Blumenthal, D-Conn., said that the Judiciary Committee, of which he is a member, is having a hard time in its Russia investigation dealing with former Trump campaign manager Paul Manafort.

"Paul Manafort has been more resistant and that is unacceptable to me," Blumenthal said on MSNBC on Thursday, comparing him to Donald Trump Jr., another person of interest in the investigation. He added, "I think that we will have to use subpoenas for a number of the witnesses we'll want to hear from."

The Judiciary Committee, along with other congressional panels and the special counsel, are looking into Russian interference in the 2016 election and whether the Trump campaign coordinated with the Kremlin.

Ultimately subpoena power rests with the chairman, who is Sen. Chuck Grassley, R-Iowa.

A subpoena had been sent by the committee to Manafort in July compelling him to testify, but that was retracted days later when he agreed to hand over documents to the committee and negotiate some sort of interview.

Earlier in the week, the committee held a 10-hour interview with Glenn Simpson, the former reporter for the Wall Street Journal whose firm helped compile the infamous "dossier" about President Trump's campaign team and Russia. Grassley said at a town hall on Wednesday that his panel will vote on whether to release the transcripts.

Grassley suggested that it may be of greater interest to the public to have Manafort, along with Trump Jr. and Simpson testify on-the-record before his committee, something which Blumenthal echoed on Thursday.

Trump Jr., who has become a central figure the Russia probe investigators over the meeting he set up with a Russian lawyer last year, has agreed to speak with the Judiciary panel in private.

Blumenthal said that he anticipates Trump Jr. to come before the staff or senators themselves "very soon."