Sen. Lindsey Graham, R-S.C., is expected to unveil a resolution Thursday condemning House Democrats' impeachment inquiry and their decision to conduct the initial investigation behind closed doors.

Graham, chairman of the Senate Judiciary Committee, plans to introduce the resolution, co-sponsored by Senate Majority Leader Mitch McConnell, R-Ky., on Thursday afternoon, according to a press release from Graham's office. The release slams the inquiry as "illegitimate," echoing arguments made by the White House and President Donald Trump as part of the administration's near-universal resistance to cooperating with a probe it has deemed invalid.

Democrats have so far conducted the impeachment investigation behind closed doors with a number of key witnesses in the Ukraine case, prompting complaints from Republicans that the inquiry was being done in "secret."

House rules allow only members of the committees holding the hearings to participate in depositions. In the case of the impeachment inquiry, those are the House Intelligence, Foreign Affairs and Oversight committees. Nearly a quarter of the GOP conference — 47 Republican members — are already able to attend the depositions.

Democrats argue that there is precedent stemming from the Nixon and Clinton impeachment proceedings for holding the initial investigation behind closed doors. They also have said they don't want witnesses coordinating their testimony.

Graham's planned resolution comes a day after a group of House Republicans stormed a secure room, known as SCIF, in the basement of the Capitol Visitor Center, where lawmakers on the three House committees were preparing to question Pentagon official Laura Cooper as part of the impeachment inquiry.

Those Republican members protested what they have said is an unfair process by violating House rules and staging a sit-in-type of demonstration that delayed Cooper's testimony by five hours. Trump thanked them Thursday morning on Twitter.

“Thank you to House Republicans for being tough, smart, and understanding in detail the greatest Witch Hunt in American History," Trump tweeted Thursday. "It has been going on since long before I even got Elected (the Insurance Policy!). A total Scam!"

On Wednesday, Graham originally criticized the breach of a secure room as "nuts," according to The Associated Press, before later tweeting, "Correction: I was initially told House GOP took the SCIF by force — basically like a GOP version of Occupy Wall Street. Apparently it was a peaceful protest. Big difference. I understand their frustration and they have good reason to be upset."

In response to a question about whether he had been pressured by Trump to ramp up hearings and oversight in response to the House's inquiry, Graham told reporters that while "people in our base are frustrated" that he is not doing more, "the solution I think is not to create chaos in the Senate."

While Republicans on the Hill increase efforts to counter the inquiry, the White House has been struggling to come up with a strategy of its own — a month into the impeachment process.

Trump’s senior adviser and son-in-law, Jared Kushner, has been searching for someone he can put into a chief strategist role who would be singularly focused on the administration’s internal and external response to impeachment, according to people familiar with the discussions.

That would create a clear line of command over who is leading the efforts and help coordinate the response between the White House and other departments pulled into the inquiry, which now includes the State, Energy and Defense departments.

But after several weeks of discussion about the role, it has yet to be filled.

Instead, there has been an internal tug-of-war between the White House’s top lawyer, Pat Cipollone, and acting chief of staff Mick Mulvaney, who have both sought to take control of the response and also have other responsibilities well beyond impeachment to focus on, the sources said.

That's left surrogates with a confused mixture of talking points. Even Mulvaney was unable to stick to a straight message during a press briefing this month when he acknowledged that the president held up aid funding to Ukraine until the country agreed to investigate 2016 election interference, a statement he walked back hours later.

That has frustrated allies outside the White House as well. Former White House strategist Steve Bannon has started a radio program offering advice to Trump on how to respond to the impeachment onslaught. The program, called the War Room, was first reported by The New York Times.

The president expressed frustration this week with his party for not fighting the investigation harder.

“Republicans have to get tougher and fight,” the president said Monday during a Cabinet meeting at the White House after calling the inquiry a “phony investigation.”

“We have some that are great fighters, but they have to get tougher and fight because the Democrats are trying to hurt the Republican Party for the election, which is coming up, where we’re doing well.”

While he criticized Democrats and called them “vicious” and “lousy politicians,” the president also praised their ability to stay united and “stick together,” which he suggested Republicans aren’t able to do.

“They don’t have Mitt Romney in their midst,” he added, referring to the Utah senator's recent criticism. “They don’t have people like that. They stick together."

Intelligence Chairman Adam Schiff, D-Calif., whose panel has taken the lead in the impeachment inquiry, suggested that the Republican members' tactics were in response to the testimony Tuesday of Bill Taylor, the top U.S. diplomat in Ukraine.

According to remarks obtained by NBC News and other news outlets, Taylor told impeachment investigators that Trump had directed officials to tie aid to Ukraine to demands that the country open an investigation into the family of former Vice President Joe Biden and the 2016 election, a potentially serious blow to Trump's repeated denials of a quid pro quo.

"Clearly the White House was devastated by yesterday's testimony, and these witnesses have been willing to defy the administration and follow the law and come testify," Schiff said. "So, the president's allies are trying to stop them through other means, but they won’t be successful."