(CNN) Gina Haspel, President Donald Trump's pick to be the next CIA director , says in a new letter that the CIA should not have conducted then-President George W. Bush's interrogation and detention program where waterboarding and other brutal interrogation tactics were used on detainees.

"While I won't condemn those that made these hard calls, and I have noted the valuable intelligence collected, the program ultimately did damage to our officers and our standing in the world," Haspel wrote in the letter, which was obtained by CNN. "With the benefit of hindsight and my experience as a senior agency leader, the enhanced interrogation program is not one the CIA should have undertaken."

Haspel's written comments go further than the statements she made during her public confirmation hearing last week. At the hearing, she said she would not permit the CIA to resume an interrogation program, but she also would not condemn the CIA's post-9/11 interrogation program beyond saying that the CIA was not prepared to run the program and she supported the "stricter moral standard" that is now the law.

Haspel has been more forthcoming with senators in their private meetings and in the classified hearing she held with the Senate Intelligence Committee last week. A source with knowledge of the classified session told CNN that Haspel told senators she did believe torture is immoral, but she did not want to be seen as publicly criticizing her colleagues at the CIA.

Haspel also told Warner that it "was a mistake not to brief the entire committee at the beginning" of the interrogation program.

"CIA needs to have consensus from members of the oversight committees who make decisions on behalf of the American people as their elected representatives on activities that can't be made public," she wrote.

Haspel's confirmation has been a difficult fight because of her role in the interrogation program, which critics say amounted to torture. Haspel is facing criticism for her supervision of a black site in Thailand where harsh interrogations were conducted along with her role in the destruction of CIA interrogation tapes.

In the letter, Haspel said she would "refuse to undertake any proposed activity that is contrary to my moral and ethical values."

"As I was able to describe in detail during the classified session, in my role as Deputy and now Acting Director, every operation I review must not only meet those high standards, the activity must also be consistent with CIA's mission, expertise and the law," Haspel wrote. "I do not and would not hesitate to reject a proposal that fails to meet this threshold."

Haspel's letter comes ahead of a committee vote on her nomination on Wednesday — in which Warner is one of the Democrats still undecided — and appears to be an effort to persuade Democrats still on the fence.

Warner was one of the Democrats still undecided on her nomination when Haspel sent the letter Monday, but the Virginia Democrat then said in a statement Tuesday he would back her nomination , a key vote that all but ensures she has enough votes for confirmation.

Warner told reporters before he announced his vote that the letter "better reflected" what Haspel has said in private conversations. "Her letter was closer to some of the individual conversations I've had with her," Warner said.

Haspel's critics argue she still didn't take any responsibility for her role in the interrogation program or state whether she believed it was immoral.

"She thinks something like this is going to whitewash a confirmation," said Sen. Ron Wyden, an Oregon Democrat.

Haspel is likely to clear the Senate Intelligence Committee vote with the support of Democratic West Virginia Sen. Joe Manchin, in addition to Warner. A Senate floor vote is likely next week, though it's possible it could be as soon as Thursday. Four Democrats -- Manchin, Warner, Indiana Sen. Joe Donnelly and North Dakota Sen. Heidi Heitkamp -- have publicly said they will back her nomination.

Two Republicans, Sens. Rand Paul of Kentucky and John McCain of Arizona, say they are opposed to Haspel's confirmation, although McCain is unlikely to be in Washington for the vote while he battles brain cancer. There are still numerous undecided Democrats, many of whom are up for reelection in states that Trump won in 2016.

Republican Sen. Jeff Flake of Arizona is also still undecided. Flake said he has spoken to McCain, who was tortured as a prisoner of war in Vietnam, about the nomination last week, and is still weighing Haspel's nomination.

"His voice means more than anybody else on this," Flake said of McCain. "But my vote is my own and I'll continue to consider it."

Flake has asked the Justice Department to make available to all senators a classified Justice Department report from John Durham, the special prosecutor who investigated the tapes destruction but did not press any charges. The report summary was made available to the Senate Intelligence Committee members, but not the full Senate.

"I'd like to see the summary, but I understand them not wanting to provide it too, I do understand their arguments there," Flake said Monday.

Several Democrats say the Trump administration has failed to properly provide senators and the public with a full accounting of Haspel's record, including the Durham report and other CIA records.

Asked about the Durham report, Wyden said Tuesday there are "holes" in her account that senators need to see before voting on her nomination.

But Senate Intelligence Committee Chairman Richard Burr, a North Carolina Republican, rejected those calls Monday. "No, no," he said when asked about making the report available to the full Senate, before walking into the Senate chamber.

Warner would not say Monday whether he thought the report should be provided to the full Senate, beyond saying that he "would like as much transparency as possible."

"This is an area where the Justice Department has never shared this kind of information in the past," Warner said.