In choosing John Brennan to be the next director of the Central Intelligence Agency, President Barack Obama is turning to a trusted deputy and architect of the administration's unconventional, covert war against radical adversaries.

A 25-year CIA veteran who joined Mr. Obama's 2008 presidential campaign after leaving the agency, Mr. Brennan straddled the counterterrorism policies of two administrations, drawing on his experience serving under President George W. Bush when he became Mr. Obama's counterterrorism chief. Indeed, Mr. Brennan may be responsible for much of what both friends and foe cite as continuity in executing the war on terror.

That careful navigation from a Republican to Democratic administration speaks to what colleagues say is an apolitical approach to his work, but it also has sown distrust among some members of both parties. On Monday, civil-liberties groups raised concerns about his role in Bush administration counterterrorism programs and the Obama administration's drone program, while Republicans questioned his forthrightness with Congress.

Mr. Brennan, 57 years old, was named to the CIA job four years after he withdrew his name from consideration for the post at the outset of Mr. Obama's first term. At the time, he had come under heavy criticism from liberals for what they thought was his responsibility for the Bush administration's enhanced interrogation of terror suspects, including tactics such as waterboarding. Mr. Brennan and his colleagues at the time say he played no role in the program.

Avoiding a confirmation battle, Mr. Obama instead appointed Mr. Brennan to be his top adviser on counterterrorism and homeland security, which put him in a position in which he met daily with the president and frequently delivered bad news.