Haspel is a career CIA officer who has spent much of her time at the agency working under cover.
Donnelly is up for re-election in a state Trump won handily in 2016. Trump nominated Gina Haspel as the new Central Intelligence Agency director, and her predecessor Mike Pompeo has been confirmed as the new Secretary of State.
Brutal interrogation practices are banned under United States law, but debate on the issue has resurfaced during Haspel's confirmation process because she was once involved in the CIA's interrogation program.
Donnelly announced that he will be voting to confirm Haspel after a " tough, frank, and extensive discussion".
In an interview Thursday on Bloomberg Daybreak, Portman predicted "a tight vote" on the Senate floor, adding it was unfortunate "because I think she deserves more support than she's than she's going to get".
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Donnelly voiced his support for Haspel on Twitter Saturday morning, saying he believes Haspel learned from the past and has the experience needed as the USA faces "dynamic and challenging security threats". "I have serious reservations about this nominee and I will oppose her", Sanders said.
In her nomination hearing, Haspel refused to give clear answers.
Sen. Jeff Flake, R-Ariz., is the only other Republican who is publicly considering opposing Haspel - in part because of his respect for McCain, a prisoner of war during the Vietnam conflict.
Haspel was chief of a base at a black-site prison in Thailand in 2002, where techniques such as waterboarding were used on terror suspects. "Importantly, Ms. Haspel expressed to me her commitment to be responsive to congressional oversight and to provide her unvarnished assessment - both to members of Congress and the president". The president attacked the senator this week, at a campaign-style rally in Elkhart, calling him "Sleeping Joe" and criticising his record in Washington, which Trump said made Donnelly, who was elected in 2012, an "incredible swamp person". He said Haspel "acted appropriately".
Democratic Sens. Martin Heinrich from New Mexico and Kamala Harris from California also admonished Haspel personally on Wednesday, for being seemingly evasive during questioning at the hearing. Rand Paul of Kentucky, has announced his opposition to her nomination. His opponent will be the state attorney general, Patrick Morrisey, who this week came through a Republican primary featuring the ex-coal boss and convicted felon Don Blankenship, whom Trump rejected as unelectable.
Haspel delivered an order from her superior to other agency officials to destroy videotapes of the interrogations, including the waterboarding of one of the top al-Qaeda operatives.