WASHINGTON — John Brennan joined the CIA as a young man after responding to a newspaper want ad. Now, at 57, he could become its next director.

President Barack Obama on Monday nominated Brennan, his chief counterterrorism adviser, to lead the CIA, praising his pick as someone already closely connected to the agency and willing to fight on its behalf.

“John knows what our national security demands: intelligence that provides policymakers with the facts, strong, analytic insights and a keen understanding of a dynamic world,” Pres. Obama said at the White House.

Brennan worked for the CIA for more than two decades and has been Pres. Obama’s assistant for counterterrorism and homeland security since 2009. In that role, Pres. Obama said, Brennan worked closely with many government agencies.