McCain acknowledged that Haspel is a patriot who has devoted her life to serving the country, but he pointed out that her role in overseeing the use of torture by Americans is "disturbing".
In the letter, Haspel said she would "refuse to undertake any proposed activity that is contrary to my moral and ethical values".
Last week, Haspel said she would not permit the spy agency to restart any kind of harsh detention and interrogation programmes.More news: Incremental North Korean denuclearisation will be unacceptable - South Korea official
Haspel, a Central Intelligence Agency veteran, ran a Central Intelligence Agency black site that conducted "enhanced interrogation" techniques that are now widely considered to be torture, The Hill noted. "I'm not one of those persons who calls it torture, an bad lot of people do".
The vote will take place in a closed session, and Haspel is expected to receive "yes" votes from at least nine of the committee's 15 members - gathering support from all eight Republicans and at least one Democrat, Sen.
"Gina Haspel was intimately involved and should not lead the agency", she said. Mark Warner, D-Va., the vice chairman of the Senate Intelligence Committee, which plans to vote Wednesday on Haspel's nomination. Joe Donnelly are the two Democrats who have publicly said they will back her nomination.
Late last week, Arizona's other Republican senator and frequent Trump White House critic, Jeff Flake, also declared himself still undecided.More news: Jeffress Prays 'For the Peace of Jerusalem' After Being Called 'Bigot'
McCain is known as a tough wrangler of votes, and his credibility on the issue of torture is widely respected among his colleagues on both sides of the aisle.
The U.S. Senate Intelligence Committee will reportedly hold the vote behind closed doors, which is its traditional procedure.
"Over the previous year I've had the opportunity to work with Ms. Haspel in her role as Deputy Director, and I have always found her to be professional and forthright with the Intelligence Committee".
Asked about the Durham report, Sen.More news: Retailer Gap apologises for T-shirts with 'incorrect' map of China
But Senate Intelligence Committee Chairman Richard Burr, a North Carolina Republican, rejected those calls Monday.