Brennan pledged, if confirmed, to "make it my mission to ensure that the CIA has the tools it needs to keep our nation safe and that its work always reflects the liberties, the freedoms, and the values that we hold so dear."
Brennan was at the president's side during some of the most significant security incidents during his first term.
The White House last week released a photograph of him briefing Obama on the elementary school massacre in Newtown, Connecticut. He is also visible in an iconic photo of top officials at the White House monitoring, in real time, the U.S. commando raid that killed al Qaeda leader Osama bin Laden in Pakistan in 2011.
Brennan is praised by former CIA officials who have worked with him. "John is a great choice - highly experienced, extremely dedicated, a person of integrity," said John McLaughlin, former acting CIA director.
But, by choosing him, Obama has given both liberals and conservative Republicans an opportunity to re-open the debate over Bush administration interrogation policies.
In a 2007 CBS television interview, while Brennan was out of government, he appeared to assert that enhanced interrogation techniques had produced useful information. "There have been a lot of information that has come out from these interrogation procedures that the agency has in fact used against the real hardcore terrorists. It has saved lives," he said.
Inside the CIA, where career intelligence officers consider it a point of pride to be above politics, employees will want to see whether Brennan's time at the White House has made him a more political figure.