(CNN) -- 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.
Brennan was intimately involved in the run up to the raid on the Osama bin Laden compound in May 2011.
Bergen interviewed Brennan and other administration officials for his book, "Manhunt: The Ten Year Search for bin Laden, from 9/11 to Abbottabad."
He tells the story about one particular debate on whether bin Laden could be at the compound.
"At one point the analysts said whoever is living in this compound has a dog. And of course, very observant Muslims don't have dogs. But Brennan has been on the bin Laden account himself for 15 years, and he remembered that bin Laden in fact had a dog when he was living in Sudan."
Brennan supported the decision to go ahead with the raid even though it was not certain that bin Laden was in fact there.
In a briefing after the assault, Brennan got into some hot water for implying that bin Laden was armed -- he was not -- and for suggesting that bin Laden used a human shield -- he did not.
"There was a female who was, in fact, in the line of fire that reportedly was used as a shield to shield bin laden from the incoming fire," Brennan told reporters.
More recently, the White House has been criticized by some Republicans for the leaks of sensitive information on counterterrorism operations that made the administration look good during the presidential campaign. Brennan has called the leaks devastating and vehemently denied that administration officials were involved.
The controversy over leaks, the drone program and his position on the CIA's interrogation and detention program could make for some rough moments during Brennan's confirmation hearings.
The fact he is coming from the West Wing and frequently talked about the president's views on camera could make him political fodder, Frances Fragos Townsend, CNN's national security contributor, told Security Clearance.
"John came out time and again and did a lot of press early on that angered Republicans on (Capitol) Hill," Townsend said. "Do I think Republicans will treat him not as a former career professional? He's got to expect that the Hill is going to treat him as a political person who is fair game now, rightly or wrongly."
None of the former officials and experts CNN spoke to believes Brennan's nomination would be upended by any of the criticisms of him.
Harlow thinks Brennan is a hard-working, no-nonsense type with the experience to run the agency.
"John's the kind of guy who is the catcher on the team that gets all dinged up, takes his hits but gets the job done," Harlow said.
CNN's John Berman contributed to this report.