Warner, of Virginia, is the third Democrat to say he'll vote for Haspel.
She is expected to be confirmed by the full Senate as soon as next week, although that vote likely also will be close.
"The United States must be an example to the rest of the world, and I support that", she said in her letter.
She had her testimony in front of the Senate Intelligence Committee last week, which was widely seen as a positive performance.
Haspel testified at a Senate hearing that torture does not work as an interrogation technique and that as director her strong "moral compass" would ensure she did not carry out any administrative directive she found objectionable. So far, just two Democrats in the Senate have said they plan to vote in favor of Haspel. Though she vowed during her testimony never to restart a detention and interrogation program and said she would obey current law, she also refused to condemn the agency's defunct torture program and insisted that it had been legal at the time.
"I have learned the hard lessons since 9-11", Haspel wrote.
Gap apologises for China map T-shirt that didn't show Taiwan
It also apologised for "this unintentional mistake and is now conducting internal investigations to quickly rectify this mistake". Mercedes-Benz said sorry for quoting the Dalai Lama on social media.
Most Republicans are expected to back Haspel, although Sen.
Such unusual move former military decided because of suspicions about the possible involvement Haspel to use enhanced interrogation techniques, in other words, the torture of suspects.
Both Donnelly and Manchin are running for reelection this year in states that President Trump carried easily in 2016, and Donnelly, in particular, was being attacked for his supposed opposition to Haspel as late as Thursday, when Trump spoke at a campaign rally in Elkhart, Indiana and called for a vote for his Republican opponent, millionaire businessman Mike Braun. John McCain of Arizona is absent.
"While I trust her word, I will also verify, helping to ensure Congress conducts robust oversight of the Central Intelligence Agency under her leadership", she added.
As a member of the Senate Foreign Relations Committee, Mr. Paul's opposition nearly triggered the panel to report Mr. Pompeo out of the committee unfavorably before the full Senate vote. It was President Obama who decided that John Brennan, Jose Rodriguez, Gina Haspel and others involved in the torture program should not face prosecution.