Washington (CNN) Acting Attorney General Matt Whitaker disregarded the advice of a Justice Department ethics official to step aside from overseeing Robert Mueller's Russia investigation.

Whitaker never sought a formal recommendation about whether he needed to recuse, but he received guidance on his options and the applicable rules during three meetings with ethics officials and multiple discussions with his own advisers, according to a senior department source and a letter from the Justice Department to Congress Thursday night.

The decision to make was Whitaker's alone and came the same day news emerged that Trump's nominee to take the permanent job, Bill Barr, wrote the Justice Department last year to argue against the Mueller investigation, raising concerns on Capitol Hill that the President is selecting leaders based on their alignment with his critical view of the Russia probe and will seek to undercut the special counsel.

Former Attorney General Jeff Sessions had recused himself from the Mueller investigation in favor of Deputy Attorney General Rod Rosenstein -- to the constant annoyance of President Donald Trump -- because Sessions had been an active participant on the Trump campaign.

Some of Whitaker's comments about Mueller in 2017 mirrored Trump's complaints. In one instance, speaking on the "Rose Unplugged" radio program in August 2017, Whitaker said the appointment of Mueller was "ridiculous" and it "smells a little fishy."

"For whatever reason, Rod Rosenstein determined that the Department of Justice couldn't handle this in their ordinary course of work, which I think was ridiculous," Whitaker said. "So I think it smells a little fishy, but I just hope it doesn't turn into a fishing expedition, because I will be one of [the people] jumping up and down making sure the limitations on this investigation continue because that's the way it's supposed to be."

While ethics officials said there was no legal conflict of interest that would bar Whitaker from overseeing Mueller, the Justice Department letter states, it could look bad in the eyes of the public.

"If a recommendation were sought" from ethics officials, the letter states, "they would advise that the Acting Attorney General should recuse himself from supervision of the Special Counsel investigation because it was their view that a reasonable person with knowledge of the relevant facts likely would question the impartiality of the Acting Attorney General."

While the process was ongoing, Whitaker was never briefed on the Mueller investigation, the senior Justice source told CNN. But Whitaker was given a heads up that Trump's former attorney Michael Cohen would plead guilty to lying to Congress about the proposed Trump Tower project in Moscow before it was publicly announced.

It is expected he will be briefed as acting attorney general now that he has stated his position on recusal.

Rosenstein's office is still managing the investigation on a day-to-day basis, as CNN has previously reported.

Whitaker's thinking

A Justice official initially told CNN that Whitaker was advised that he did not need to recuse. But a different picture soon emerged from the senior Justice official close to the process, who described how ethics officials noted there could be an appearance of a conflict based on Whitaker's past public comments about the investigation.

The ethics official did tell Whitaker's office the verdict was a "close call," according to a source and the letter to Congress.

Whitaker was of the mind that if it were deemed a close call, he did not want to bind his successors in a situation where there was only an appearance, not an actual legal conflict, according to the senior DOJ official close to the process.

In explaining Whitaker's thinking, the letter noted that Whitaker has not made public comments about the investigation for 16 months, Whitaker has "a lot of respect" for Mueller and said Mueller would "only go after legitimate targets."

The letter also pointed to comments Whitaker made to South Carolina Republican Sen. Lindsey Graham, that he did not see a reason to fire the special counsel or believe the investigation "breached any Department guidelines."

Attention turns to Barr

Meanwhile, Democratic lawmakers are turning their attention to William "Bill" Barr, whom Trump has nominated to be the next attorney general on a permanent basis.

If confirmed, Barr would oversee the Mueller investigation, though his recently surfaced belief that Trump's interactions with ex-FBI Director James Comey would not constitute obstruction of justice triggered an outcry from Democrats Thursday.

Senate Minority Leader Chuck Schumer demanded Trump withdraw the nomination.

California Democratic Sen. Dianne Feinstein said in a statement that the Barr memo was "very troubling" and contended it meant Barr had argued "the President is above the law."

"We need answers as to why Barr proactively drafted this memo and then shared it with the deputy attorney general and President Trump's lawyers," read the statement from the top Democrat on the Senate Judiciary Committee.

Virginia Democratic Sen. Mark Warner, the vice chairman of the Senate Intelligence Committee, told CNN he wants President Trump to "withdraw" the nomination of Barr in the aftermath of news of the memo showing Barr raising concerns about the Mueller probe, and he also sharply criticized the decision by Whitaker to not recuse from the Mueller probe.