"My son's a good person," Trump said, "He works very hard. The last thing he needs is Washington, D.C. He would rather not ever be involved."
He also said he was leaving the question of whether Mueller should testify to Congress up to his "very great attorney general," William Barr.
But just as soon as he said he said he won’t get involved in the decision, the president then launched into an extended argument about why there should be no further investigations stemming from the Russia probe, saying there was “no collusion and essentially no obstruction.” Trump has not previously used the qualifying description "essentially."
The president did not say whether his son should fight the subpoena but he did say that he had previously testified "for a massive amount of time."
Trump also singled out the Republican chairman of the Senate committee that subpoenaed Trump Jr.
"I saw Richard Burr saying there was no collusion two or three weeks ago. He went outside and somebody asked him. "No, there's no collusion. We found no collusion," he said about Burr.
On Capitol Hill, Republicans blasted Burr, who is not running for re-election, for signing off on the subpoena.
GOP Sen. Rand Paul took a major swipe at Burr, telling reporters that the subpoena is an “overzealous persecution of the president’s family” and that it couldn’t have happened “without Republicans being complicit in it.”
"At some point this is is not about finding facts, this smacks of politics," Sen. John Cornyn, R-Texas, a committee member, told reporters Thursday morning. "And I think we have an important job to do to try to keep the intelligence committee out of politics..."
"I can understand his frustration at being asked to come back after having cooperated for such a long period of time, seeing now that the Mueller report has concluded, sort of wondering what the purpose of this is," Cornyn said of Trump Jr.
Other Republicans also expressed their sympathies for Trump Jr.
"I think about the time that he spent hours with our staff and testimony that's recorded, and then now everything post the Mueller report coming out, and its conclusions of no collusion or any crime," Sen. Chuck Grassley, R-Iowa, said. "What more is there to do now?" he added.
The top Democrat on the committee – Sen. Mark Warner – told reporters that he commends Burr for his work on the committee and for working with him in such a bipartisan manner.
“Some of the criticisms out there are not the first time the chairman or I have received criticisms over this investigation,” Warner told reporters.
“I am proud of this committee because we're the only committee that is still bipartisan, still following the facts. And we're going to continue that process,” he said.
GOP Sen. Susan Collins of Maine said that while she can’t talk about the subpoena, she has Burr’s back.
“I believe that Senator Burr has been a very good chairman, has done an excellent job conducting this investigation and has worked very closely with the ranking Democrat on the committee,” Collins said.
Trump Jr. has previously met with three congressional committees, including the Senate Intelligence Committee in December of 2017, with whom he met for more than nine hours as part of their investigation into Russian interference in the 2016 presidential election. He met with the Senate Judiciary Committee in September of 2017 for more than five hours and with the House Intelligence Committee for more than eight hours in December of 2017.
It is unclear why the committee wants Trump's son to return to testify.
A source close to Trump Jr. said there was an agreement between the president’s son and the committee that he would only have to come in and testify once as long as he was willing to stay for as long as they’d like, which the source said he did.