Matthew Whitaker, left, talks with fellow U.S. Senate candidate Sam Clovis during an April 2014 debate in Johnston, Iowa. (Charlie Neibergall/AP)

The top Democrat on the Senate Intelligence Committee is intensifying his calls for acting attorney general Matthew G. Whitaker to recuse himself from supervision of the special counsel’s probe, citing Whitaker’s friendship with a key witness in the investigation.

“There is a blatant conflict of interest here that demands recusal,” Sen. Mark R. Warner (Va.), the ranking Democrat on the Senate Intelligence Committee, said in an interview.

“[Sam] Clovis is more than just a witness,” Warner said. “He was the supervisor of the Trump Foreign Policy Advisory Committee, which included a cast of rogue characters.”

Clovis, who served as national co-chairman of President Trump’s campaign, was a key contact for two of Trump’s foreign policy advisers who have come under scrutiny: George Papadopoulos, who has pleaded guilty to lying to the FBI, and Carter Page, the subject of a secret intelligence warrant.

Papadopoulos and Page have acknowledged contact with Russians during the campaign, but they said that the contacts were appropriate and that they have been victimized by overzealous investigators.

Clovis has been interviewed by special counsel Robert S. Mueller III’s team, appeared before the grand jury probing Russian interference in the 2016 election and been called before congressional committees.

The Iowa Republican activist has described the acting attorney general as a “dear friend” and a longtime political ally.

Clovis and Whitaker have been close since both made unsuccessful U.S. Senate bids in 2014. Later that year, Whitaker chaired Clovis’s bid for Iowa state treasurer. In a recent interview with Reuters, Clovis said Whitaker was a “sounding board” for him when Clovis worked for Trump’s campaign. After Whitaker was tapped by Trump to serve as acting attorney general last week, Clovis said he texted him congratulations.

Neither Clovis nor his attorney, Victoria Toensing, responded to a request for comment.

But in an interview with The Washington Post last week, Clovis dismissed the suggestion that their relationship raised questions about Whitaker’s ability to oversee the Russia probe.

“It’s not relevant, and Matt has high integrity. I’m very happy for him, and he’ll do a fantastic job,” Clovis said.

Justice Department spokeswoman Kerri Kupec said the acting attorney general “is following the processes and procedures of the Department of Justice on all matters.” She declined to comment on Whitaker’s relationship with Clovis.

Warner’s comments came as other Democratic lawmakers pushed back against Whitaker’s appointment, saying it violated laws governing Cabinet vacancies.

Earlier this week, top Democrats from the House and Senate, including Warner, sent a letter to the Justice Department’s ethics chief demanding to know whether he advised Whitaker to recuse himself from the special counsel’s probe into Russia’s interference in the 2016 campaign.

In their letter, the Democrats argued that he has a “clear bias” against the investigation, citing comments he made before joining the Justice Department in which he was critical of the probe.

Warner said in an interview that the most powerful argument for recusal is Whitaker’s longtime relationship with Clovis, a man he describes as a central player in the still unfolding Russia drama.

“That would appear — even to my foggy memory of legal ethics classes — to smack of a classic conflict of interest,” Warner said. “We are talking about the acting attorney general of the United States. This is a situation that demands recusal.”

Other Democratic lawmakers have echoed that call.

The Clovis-Whitaker relationship “is one of many conflicts of interest that explains why he should recuse himself from overseeing Mueller and his investigation,” Rep. Joaquin Castro (D-Tex.), a member of the House Intelligence Committee, said in a statement. Earlier in the week, the incoming chairman of that panel, Rep. Adam B. Schiff (D-Calif.), said Whitaker’s ties to Clovis are troublesome and “cause for recusal.”

Republican senators have defended Whitaker. During an interview Thursday, Sen. Lindsey O. Graham (R-S.C.) rejected the idea that recusal was necessary. He said that he had spoken to Whitaker and that the acting attorney general “believes he doesn’t have a legal or factual reason to recuse,” and also has no plans to interfere with Mueller’s investigation.

“He says he is going to follow regular order. Regular order to me means he’s not going to do anything crazy,” Graham said, adding that Whitaker told him he has not had any interaction so far with Mueller.

Clovis was one of the earliest supporters of Trump’s presidential bid. He was named national co-chairman of Trump’s campaign after endorsing the celebrity businessman in August 2015 and held an affiliation with the campaign through Election Day.

In his role at the Trump campaign, Clovis helped bring in two advisers who have been key in the Russia investigation: Papadopoulos and Page.

Page was an energy consultant who had lived in Moscow and advocated warmer U.S. ties with Russia. Papadopoulos encouraged Trump to meet with Russian President Vladi­mir Putin during the campaign, according to emails and congressional testimony.

Page told the House Intelligence Committee in November 2017 that he was connected with Clovis in January 2016 by campaign manager Corey Lewandowski after he volunteered his services. Page later had breakfast with Clovis in Northern Virginia to discuss joining the campaign, he told the committee.

Papadopoulos told prosecutors that he spoke to Clovis before he was named a foreign policy adviser, according to court documents.

According to court filings, Papadopoulos said that Clovis told him that building better relations with Russia was Trump’s principal foreign policy goal.

Clovis has denied making those remarks.

Papadopoulos kept his supervisor apprised of his interactions with a London-based professor and a Moscow think tank director and that those contacts could help the candidate secure a meeting with Putin, according to court documents. The Washington Post has identified Clovis as the supervisor.

Clovis responded that he would “work it through the campaign,” adding, “Great work,” according to filings.

Devlin Barrett and Josh Dawsey contributed to this report.