Current and former diplomats and leading political figures have heaped praise on Hagel in recent days, defending his record and highlighting his bipartisan values.
"No one has been more steadfast in supporting America's commitment to Israel's security than has Senator Hagel," wrote a group of six former political and diplomatic officials, including former National Security Advisors Brent Scowcroft and Zbigniew Brzezinski, in a public letter to Obama.
"Our polarized political life is much in need of leaders with the kind of bipartisanship and independence of conscience and mind that Chuck Hagel's service to our country has exemplified," they wrote.
Former U.S. ambassador to Iraq and Afghanistan Ryan Crocker wrote in a "Wall Street Journal" editorial that Hagel "knows the leaders of the world and their issues" and "has an unbending focus on U.S. national security."
"Chuck Hagel is pro-gay, pro-LGBT, pro-ending 'don't ask, don't tell.' The only problem is that no one asked him his views lately," wrote Steve Clemons, an editor for The Atlantic and Hagel ally who has spoken extensively with the former senator, in a column.
Hagel is in many ways an ideal pick for Obama, giving nod to bipartisanship while appointing someone with a demonstrated commitment to veterans of Iraq and Afghanistan and to retooling and economizing the Pentagon bureaucracy for the future.
Obama's top counterterrorism adviser since 2009, Brennan faces less uncertainty going into his confirmation hearings than Hagel.
As a career national security administrator, Brennan has a history with the agency he's been nominated to lead going back to the Cold War. After joining the CIA as an intelligence director in 1980, the New Jersey native held positions at home and abroad, at one point working in Saudi Arabia as a station chief.
"People here in the White House work hard, but John is legendary even in the White House for working hard," Obama said today. "He is one of the hardest-working public servants I've ever seen. I'm not sure he's slept in four years."
Since joining the White House, Brennan played a role overseeing the operation that killed Osama bin Laden. He is responsible for the president's daily security briefing and represents the administration in any terror incident.
One of his first tests came in December 2009 when a Nigerian man attempted to ignite an incendiary device aboard a jetliner landing in Detroit. Brennan defended the administration's position to try the suspected terrorist, Umar Farouk Abdulmutallab, in criminal court over a military tribunal.
"Cries to try terrorists only in military courts lack foundation," he wrote in an op-ed at the time. "There have been three convictions of terrorists in the military tribunal system since 9/11, and hundreds in the criminal justice system -- including high-profile terrorists such as [so-called "shoe bomber" Richard] Reid and 9/11 plotter Zacarius Moussaoui."
But his nomination may not be smooth sailing. In 2008 Obama attempted to name him CIA director but he withdrew when progressives attacked him for the agency's role in "enhanced interrogation" used on detainees of the War on Terror.
Brennan denied his involvement in the Bush administration policy, which opponents consider torture and outside the ethical bounds of the Defense Department, but still withdrew his name from the bid.
The White House says they do not expect enhanced interrogation to pose an issue in the confirmation hearings because as Obama's national security adviser Brennan put an end to the practice.
"He has worked to embed our efforts in a strong legal framework," Obama said of Brennan today.
ABC News' Ann Compton, Luis Martinez, David Kerley and Matthew Larotonda contributed reporting.