Sen. Dick Blumenthal (D-Conn.) said on Thursday that if Donald Trump Jr. didn't comply with a subpoena to testify before Congress that he should face prison time.

"I've said repeatedly that his answers, to questions, and I watched him, were smart-alecky, untruthful, and misleading," Blumenthal said to the media about Trump Jr.'s first testimony to Congress in 2017.

The Senate Intelligence Committee issued a subpoena to the president's oldest son on Wednesday to answer about discrepancies with the testimony of Michael Cohen, President Donald Trump's former longtime attorney and friend.

"So, I want to hear about what the truth is as to what he said to his father about the meeting in Trump Tower with Russian agents," said Blumenthal, "why he said he was only peripherally aware of the negotiations on the Moscow negotiation, when he was in the thick of it. He truly misled the Judiciary Committee, and now he needs to be held accountable, those answers about the Moscow Trump Tower, the meeting with Russian agents and other negotiations, really need to be answered."

Blumenthal was asked about what would happen if Trump Jr. didn't comply with the subpoena.

"He ought to be held in contempt, and there's no question with Donald Trump Jr. about any sort of privilege to avoid this kind of questioning," Blumenthal responded.

"If he fails to answer, he ought to be put in jail," he added forcefully.

"If he fails to comply with a lawful subpoena, he has no privilege," Blumenthal concluded, "prison is the only answer."

A report from CNN indicated that Trump Jr. was considering either pleading his Fifth Amendment right against self-incrimination or simply not appearing at all.

Here's the video of Blumenthal's threat: