If President Barack Obama's selection to lead the CIA is confirmed, it will be a homecoming of sorts.

John Brennan, the president's chief homeland security and counterterrorism adviser, spent 25 years at the Central intelligence Agency distinguishing himself as a Mideast and terrorism expert.

But moving back to Langley would be a big change.

He's spent the past four years at the White House, where he has had Obama's ear.

If there is an act of terrorism against Americans anywhere in the world or a mass shooting at home, Brennan is often the one who picks up the phone or walks into the Oval Office, day or night, to tell the president about the calamity.

He has been the president's trusted counterterrorism and homeland security aide who can be seen in photographs briefing Obama on such incidents as the mass shooting in Newtown, Connecticut, or the Times Square bombing attempt in 2010. He's with the president in the Situation Room during crisis deliberations.

Now Obama wants him to be the CIA director, and many see his time spent in the West Wing as a plus.

"He knows what the president wants from his intelligence community," said Bill Harlow, a former senior CIA official. "And he also knows how to deliver it from having worked at the agency ... There'll be no learning curve for him."

Peter Bergen, CNN's terrorism analyst, said Brennan would lose his immediate access to the president, but added, "he already enjoys such close relations with President Obama that I don't see it being a situation where he won't have the president's ear on a regular basis."

Brennan, 57, joined the CIA after responding to a want ad in the newspaper and spent 25 years there developing a deep knowledge of the Mideast and fluency in Arabic.

He was the CIA station chief in Saudi Arabia when terrorists bombed the Khobar Towers in 1996, killing 19 American servicemen. He also traveled extensively around the region developing ties with officials there.

He served a stint as President Bill Clinton's daily intelligence briefer and moved into senior management positions at the agency. He was CIA Director George Tenet's chief of staff before being named deputy executive director of the agency.

In 2003, Tenet asked him to start what would eventually become the National Counterterrorism Center, a multiagency center intended to bridge the gaps in intelligence apparent after the 9/11 terrorist attacks on the United States.

When the Bush administration passed him over as the permanent head of the counterterrorism center, he left the agency in 2005 for private business.

In 2008, he signed on to be Obama's intelligence adviser during the presidential campaign.

After the election, he was touted as the shoo-in to become CIA director, but it was not to be. Harsh attacks from critics who claimed he supported the Bush administration's policy of harsh interrogations prompted an embittered Brennan to drop out of consideration for the job he coveted.

In a letter to Obama, Brennan wrote, "It has been immaterial to the critics that I have been a strong opponent of many of the policies of the Bush administration, such as preemptive war in Iraq and coercive interrogation tactics."

But soon afterward, at Obama's request, Brennan agreed to be the president's assistant for homeland security and counterterrorism. He assumed what has turned out to be a very powerful role within the White House: the point man on all things regarding counterterrorism policy.

Last year in an interview with the Washington Post, Brennan talked about his close relationship with the president.

"Ever since the first couple of months, I felt there was a real similarity of views that gave me a sense of comfort," Brennan said. "I don't think we've had a disagreement."

Brennan has shaped the White House's strategy to aggressively pursue suspected terrorists -- dramatically escalating the use of armed unmanned aircraft, often referred to as drones -- and to kill them in the ungoverned territories of Pakistan and in Yemen. Small teams of special operations forces have been deployed to critical locations.

Civil liberties and human rights groups who refer to the missile strikes as extrajudicial killings have harshly criticized the drone program.

Brennan became the first government official to publicly discuss the use of drones in a speech last year at the Wilson Center.

"We conduct targeted strikes because they are necessary to mitigate an actual ongoing threat--to stop plots, prevent future attacks and save American lives," Brennan said.

He said the drone strikes are destroying al Qaeda, and as the terrorist threat fades, he hoped the United States would rely less on such killings.