Acting White House chief of staff Mick Mulvaney attends a ceremony marking one year of the first lady's “Be Best” campaign in the Rose Garden of the White House on Tuesday. (Shawn Thew/EPA-EFE/Shutterstock)

Acting White House chief of staff Mick Mulvaney on Wednesday criticized Republicans on the Senate Intelligence Committee for not informing him that President Trump’s son Donald Trump Jr. would be subpoenaed by the panel as part of its ongoing probe into Russian interference in the 2016 campaign.

In an interview with CBS News, Mulvaney said it was “bad form” for the Republican-led committee to subpoena Trump Jr. without giving him advance notice.

“I have no difficulty with bipartisanship, but to subpoena the president of the United States’s son and not at least get a heads-up, I thought was — let’s say — bad form,” Mulvaney said.

The Intelligence Committee is seeking additional closed-door testimony by Trump Jr., who has been a focus of several probes — including special counsel Robert S. Mueller III’s investigation — over his involvement in a June 2016 meeting at Trump Tower with a Russian lawyer who allegedly had promised incriminating information about Democratic presidential candidate Hillary Clinton.

Congressional Democrats are concerned that in his previous appearances on Capitol Hill, Trump Jr. may have lied to investigators about that meeting and whether he alerted his father that it would take place.

As negotiations over Trump Jr.’s testimony continued, Senate Intelligence Committee Chairman Richard Burr (R-N.C.) became increasingly frustrated and believed that Trump Jr. was defying the committee’s authority and not honoring his original agreement, a person familiar with the matter said.

The panel’s move has prompted some Senate Republicans to sharply criticize Burr, who has said he plans to retire in 2022.

John Cornyn (R-Tex.), the second-ranking member of Senate GOP leadership and a member of the Intelligence Committee, told reporters that he “can understand Mr. Trump’s frustration” at being asked to appear again before the panel.

Cornyn, who is running for reelection, said he was not aware the subpoena had been issued and plans to speak with Burr and Intelligence Committee members about “what we need to do to wrap up our investigation.”

“At some point, this is not about finding facts,” Cornyn said, according to CBS News. “This smacks of politics. And I think we have an important job to do to try to keep the Intelligence Committee out of politics.”

Asked to clarify, Cornyn then backtracked and said he was not accusing Burr of playing politics. A Cornyn spokesman said the senator was “saying at some point the congressional investigations smack of politics, not specifically this decision.”

Burr’s fellow North Carolinian, Sen. Thom Tillis (R), also signaled that he disagreed with the committee’s decision to subpoena Trump Jr.

“I agree with Leader McConnell: this case is closed,” Tillis said in a Thursday morning tweet. “The Mueller Report cleared @DonaldJTrumpJr and he’s already spent 27 hours testifying before Congress. Dems have made it clear this is all about politics. It’s time to move on & start focusing on issues that matter to Americans.”

Tillis later explained his thinking in brief comments on Capitol Hill. 

“We have a 400-plus page report that was determined after about $30 million of money spent, dozens of investigators, hundreds of subpoenas and inquisitions that there was no underlying crime and no obstruction,” Tillis said. “I personally believe Democrats are just trying to keep this thing alive, and it’s their latest launch point to do it.”

But when a reporter noted that it was the committee led by Burr who issued the subpoena, Tillis responded: “I think you’d have to speak to Senator Burr. I stand by my comment.”

Tillis is up for reelection in 2020 and faces a primary challenge from the right. His tweet followed a warning earlier Thursday from Charlie Kirk, the founder of the pro-Trump organization Turning Point USA. Kirk is a friend of Donald Trump Jr.

“Conservatives are watching closely how @SenThomTillis responds to his North Carolina colleague @SenatorBurr’s senseless targeting of @DonaldJTrumpJr,” Kirk said on Twitter. “Primaries will not be kind to Republicans who stand silent as government power is a abused to harass the President’s family.”

Sen. Rand Paul (R-Ky.), a leading Trump ally, has also publicly questioned Burr’s decision.

“Apparently the Republican chair of the Senate Intel Committee didn’t get the memo from the Majority Leader that this case was closed,” Paul said in a tweet.

Senate Judiciary Committee Chairman Lindsey O. Graham (R-S.C.) stressed he would not second-guess Burr, but said in his view, “Mueller’s the final word for me.” 

“If I were Don Jr.’s lawyer, I’d be reluctant to jump back into this circus,” Graham added. “It’s just crazy. Mueller spent two years and $25 million and to me, he’s the final word.”

But “we’ll leave it to Don Jr. and his lawyers to figure out what to do,” Graham said. “He’s gone to the committee, he talked extensively to the committee. It’s my understanding, it was his understanding that it’s over. So I mean, he would come once, but I’ll leave that up to the committee.” 

Senate Majority Whip John Thune (R-S.D.), the second-ranking Senate Republican, acknowledged some of the concerns from his ranks about Burr’s subpoena and its seemingly abrupt nature. 

“But I think most of our members trust him and the process that he’s put in place and the members of the intelligence committee to do this the right way and to get the report filed,” said Thune, adding that he has a lot of confidence in Burr. “It’ll hopefully give us some additional insights into kind of what the Russians were doing in trying to meddle in our election in 2016.” 

However, House Minority Leader Kevin McCarthy (R-Calif.) said bluntly: “I think they have it wrong.” 

“He's already spent more than 20 hours in this committee. It is time for this country to move forward,” McCarthy said. “I think they should readdress [the subpoena] and look at it. And we should start working on the issues that we all know [we] should be.”

Sen. Mark R. Warner (Va.), the top Democrat on the Intelligence Committee, defended the panel’s handling of its probe on Thursday but declined to discuss the details of the Trump Jr. subpoena.

“Witnesses we’ve called early in the investigation, we’ve always reserved the right to call them back,” Warner said at a breakfast hosted by the Christian Science Monitor. “Many of those witnesses, even significant ones, have come back because I think they feel an obligation to allow us to finish this effort.”

Mulvaney said in Wednesday’s CBS interview that although he was not told that Trump Jr. would be subpoenaed, he did not know whether others in the White House had been informed.

“Possible, but unlikely,” Mulvaney said, adding: “I’m not involved in the president’s — his legal matters regarding his business, his legal matters regarding his family. I don’t do that. I handle the West Wing of the government.”

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-Ky.) on Tuesday declaring, “Case closed.”

But the Senate Intelligence Committee appears to have escaped pressure from leadership to end its probe.

Mulvaney on Wednesday claimed that what McConnell’s statement “was speaking to was that the president has been exonerated,” a false claim that the White House repeatedly has made in the wake of the Mueller report’s release.

Mueller declined to say one way or the other whether Trump should have been charged, citing long-standing Justice Department guidance that sitting presidents cannot be indicted. This week, more than 450 former federal prosecutors who worked in both Democratic and Republican administrations signed a statement asserting that Trump would have been charged with obstruction if he were not president.

In the CBS News interview, Mulvaney also insisted that while the president and his son “do share the same name,” they are “two different people,” in an apparent effort to put distance between the two.

“There was no collusion and no obstruction, period, end of story,” he said. “Now it’s time to move on to the business of government. Does that mean that other individuals may or may have not done other things? I have no idea, but I think that’s what Mr. McConnell was speaking to, the implications for the president of the United States.”

He added: “There was no reason that McConnell would go to the floor to talk about Don Jr. That’s just not going to happen. He was talking about the president.”

Burr has said that he hopes the committee’s investigation will be completed by August. Asked whether that was a reasonable timeline, Warner told reporters it depends on the panel’s findings, although he acknowledged Burr has “been under pressure to get it done.”

“We started with this [idea] that we’re going to follow the truth wherever it leads,” Warner said. “I think that’s still our mantra. I think to have it done by August, that’s a goal I could support. But the remarkable thing is almost every path we’ve led to has opened up more people, more contacts, more connections.”

He said that the panel had “90 to 95 percent” of the information Mueller’s team had gathered on Russian interference efforts, but that it has also made additional discoveries not included in Mueller’s report.

“We will have other areas that will frankly be much more extensive than what Mueller had and much more descriptive about the organized ongoing effort,” he said.

Ellen Nakashima, Ashley Parker and Matt Zapotosky contributed to this report.