Haspel's comments came in a letter to Senator Mark Warner, the top Democrat on the Senate Select Committee on Intelligence, who later on Tuesday said he would support Haspel's nomination. With three additional Democrats supporting her nomination, it appears that Haspel now has the votes to be confirmed by the full Senate.
In the letter sent to Warner on Monday, Haspel also acknowledged that it was "a mistake" for the CIA not to have briefed Congress about the program at its inception.
Haspel comments went further than those she expressed publicly during her contentious confirmation hearing last week.
Last week Haspel told the Senate Select Committee on Intelligence that, should she be confirmed, she would not bring back the agency's controversial rendition, detainee, and interrogation program.
“I don't believe that torture works," she told the committee, but she stopped short of saying whether the interrogation program was "immoral" or should have ever been carried out.
"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," she wrote in a letter sent Monday to Senator Mark Warner, the ranking Democrat on the committee.
"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 continued.
Haspel, poised to potentially become the first woman to head the CIA, faced pointed questions during last week's hearing about her reported role in the CIA's "black sites" — overseas prisons the agency used to hold top al Qaeda terrorists.
In the letter to Warner, she also wrote that "it was a mistake not to brief the entire Committee at the beginning."
"Both the committee and the Agency shared the goal of obtaining the critical intelligence needed to thwart another attack," Haspel wrote. "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."
Later Tuesday, Warner acknowledged that Haspel's comments in writing had helped in his "difficult decision" to support her nomination.
"There are valid questions that have been raised regarding the Acting Director’s record, and I have been frank with Ms. Haspel that I wish she had been more open with the American public during this process," Warner said in a statement.
"However, in both our one-on-one meetings and in classified session before the Committee, I found Acting Director Haspel to be more forthcoming regarding her views on the interrogation program, which is why I asked her to memorialize those comments in writing. I also take to heart the strong support Ms. Haspel has among rank-and-file members of the intelligence community and from intelligence community leaders who served under President Obama," Warner's statement said.
In addition to Warner's key expression of support, Democratic Senator Heidi Heitkamp of North Dakota joined Senators Joseph Manchin of West Virginia and Joe Donnelly of Indiana in supporting Haspel's nomination. The three senators face tough re-election battles in states where President Trump won by large margins in 2016.
In her letter, Haspel reaffirmed that "as Director she would refuse to undertake any proposed activity that is contrary to my moral and ethical values."
She disclosed that in the classified hearing before the committee that followed the open confirmation hearing she stressed that "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.
"I do not and would not hesitate to reject a proposal that fails to meet this threshold," Haspel wrote.
ABC News' Trish Turner contributed to this report.