Former national security adviser John Bolton was advised to purge potentially biased holdovers from the Obama administration from the Trump White House, according to a top Republican in Congress.

House Intelligence Committee ranking member Devin Nunes said on Thursday that he warned Bolton, whom Democrats want to testify in the Senate impeachment trial against President Trump, of an anti-Trump spying operation by members of the National Security Council. The California congressman also said he told H.R. McMaster, who preceded Bolton, and Robert O'Brien, the current national security adviser, to take steps to cull what he called a "den of thieves."

"I have said this over and over and over again. I said this to McMaster, I said it to Bolton, and I said it to the new national security adviser, O’Brien, that they just need to get all those people out of there," Nunes told Fox News host Tucker Carlson. "If you were there and worked for Obama and you were a holdover, just, you know, get a used building somewhere on the other side of the Potomac. Just get them out of there. They have done so much damage to the presidency. I couldn’t agree with you more."

Carlson asked why it appears to be so difficult to remove these officials when a new president is elected and a new government is set up, to which Nunes replied, "I have no idea."

"That’s why I think I gave them very good advice, which is there is plenty of empty buildings, just say, 'Look, thank you for your service, but just get off the premises,' because they do nothing but spy on them," the congressman added.

Leaks have been an issue since the early days of the Trump administration, including when a memo by McMaster about stopping such disclosures was promptly leaked to the media and published. McMaster, a retired Army lieutenant general who was national security adviser from February 2017 to April 2018, later described three "categories" of people who worked at the White House, including two groups that he said posed "a danger to the Constitution."

In October, it was reported that Trump ordered a reduction in size of the National Security Council, comprising roughly 310 career government officials from the State Department, Pentagon, and intelligence agencies and meant to advise the president on foreign policy and national security matters. The move came after reports that a CIA analyst, who was at one point assigned to the White House, filed the whistleblower complaint that led to the impeachment proceedings centered on Trump's dealings with Ukraine.

Earlier this month, Bolton announced that he is willing to testify in the Senate impeachment trial if called by lawmakers. While Trump has indicated that he will block Bolton, Senate Democrats will force votes on summoning the witnesses on their list next week. Bolton has been on their wish list ever since his former aide, Fiona Hill, testified to House impeachment investigators that Bolton opposed the Ukraine pressure campaign, saying Bolton wanted no part of what he described as a "drug deal" that fellow Trump administration officials were "cooking up."

Carlson began his interview with Nunes by reporting that Army Lt. Col. Alexander Vindman is still a member of the National Security Council. The host asked how Vindman, who testified during House impeachment proceedings that he raised national security concerns about Trump's July phone call with Ukrainian President Volodymyr Zelensky, hasn't been booted from his position.

Nunes said that Vindman, his brother who also joined the National Security Council, and others have "colluded and coordinated" with the whistleblower who reported the Trump-Zelensky call and an effort to conceal details of that private conversation and others using a highly secure computer system.

Noting that Vindman has said Ukrainian officials offered him the top defense position in Ukraine, Carlson said that the Purple Heart recipient should "go work for Ukraine."