Democrats in Congress have begun to express stronger opposition to the use of drones, but on Wednesday Obama found an unlikely ally in Republican Sen. Lindsey Graham, who expressed his "100 percent" support of the use of drones against terror suspects.
Brennan will also face questions about charges that White House officials leaked details of the administration's national security policies, including its cyberattacks against Iran's nuclear infrastructure, to burnish Obama's standing as commander in chief ahead of last year's presidential election. Brennan himself has come under fire by Republican lawmakers who believe he gave the media too many details in news conferences after the 2011 killing of al-Qaida mastermind Osama bin Laden.
The Obama White House launched a sweeping investigation led by two Justice Department lawyers in response to congressional ire over the leaks. But White House officials have defended briefings given by Brennan as authorized and backed by the president himself, who they say has the ultimate authority to declassify information.
Brennan told the Senate committee in his written answers that he was questioned as a voluntary witness in the leak investigation. He also said that in his current role, he is "vigilant about not disclosing classified intelligence matters with unauthorized persons, including reporters and media consultants."
House Intelligence Committee Chairman Mike Rogers, R-Mich., who has strongly criticized the administration's release of information on its national security programs, predicted "lots of questions about leaks and detention" at the Senate hearings, but he also predicted that Brennan would ultimately be confirmed.
When Brennan joined the White House as the top counterterrorism adviser in 2009, he publicly decried the interrogation practices, saying they backfired and produced more terrorists, leading him to urge the newly elected president to stop them.
That represented an evolution from earlier statements to the media.
In a CBS News interview in 2007, Brennan acknowledged that the practices came close to torture, but he seemed to defend them. "There has been a lot of information that has come out from these interrogation procedures that the agency has, in fact, used against the real hard-core terrorists," Brennan said. "It has saved lives."
Brennan told the committee in his written responses that "a lot of information, both accurate and inaccurate, came out of interrogation sessions conducted by (the) CIA," and that he believed the techniques were legal but "counterproductive."
"These techniques would not be used again by the CIA if I were the director," he wrote.
In a letter Wednesday, Sen. John McCain, R-Ariz., asked Brennan whether there was "any record from this period of time of your alleged opposition to waterboarding" and whether he had expressed his "alleged opposition to waterboarding or other enhanced interrogation techniques to (then-CIA) director George Tenet or his successors."
McCain, a member of the Senate intelligence panel, also asked Brennan to "specify which detainees were subjected to enhanced interrogation procedures who as a result offered information that 'saved lives.'"
After 25 years at the CIA, Brennan moved from his job as deputy executive CIA director in 2003 to become director of the Terrorist Threat Integration Center, and then interim director of its next incarnation, the National Counterterrorism Center. When Bush's second term began, Brennan left government to run The Analysis Corp., which provides counterterror analysis to government agencies, from 2005 to 2008. After Obama's election, he returned to the government payroll, in 2009, as the White House counterterror adviser.
Brennan was nominated to replace CIA director David Petraeus, who resigned late last year, citing an extramarital affair.
Feinstein has signaled she will support Brennan's nomination, but congressional staffers say both she and her Republican counterparts will ask Brennan to explain publicly if he objected to the interrogation program, and whether he believes it produced any useful intelligence.