3 People Who Could Replace Chuck Hagel

Defense Secretary Chuck Hagel's resignation this morning sparked speculation about President Obama's next potential nominee, who will have to lead the Defense Department in its fight against terrorist group ISIS.

Obama's nominee will likely come from this list:

Ashton Carter, Former Deputy Defense Secretary

Dr. Ashton Carter served as the Deputy Defense Secretary from October 2011 to December 2013, where he oversaw military budgeting during a troubling financial time for the department.

As Under Secretary of Defense for Acquisition, Technology and Logistics from April 2009 to October 2011, and continuing into his job as Deputy Defense Secretary, Carter was instrumental in supplying US troops based in Iraq and Afghanistan with Mine-Resistant Ambush-Protected vehicles, or MRAPs, a counteract to IEDs.

He also served in the Clinton administration as Assistant Secretary of Defense for International Security Policy.

Carter graduated summa cum laude from Yale University with degrees in physics and medieval history, and received his doctorate at University of Oxford. Before Dr. Carter got involved in politics, he was chair of Harvard's International and Global Affairs. Dr. Carter is also the co-author of several books on national security and defense.

Michele Flournoy, Former Under Secretary of Defense

Michele Flournoy, former Under Secretary of Defense for Policy, became the Pentagon's highest ranking woman ever when she was confirmed by the Senate in 2009. A so-called "key architect" of President Obama's national security policy, Flournoy served for three years as the principal adviser to then-Secretary of Defense Leon Panetta.

In 2012, she stepped down from her Defense Department position to become a national security adviser on Obama's re-election campaign. She later joined the Boston Consulting Group as a senior adviser to the public sector.

Flournoy earned a bachelor's degree in social studies from Harvard University and a master's degree in international relations from Balliol College at the University of Oxford.

Sen. Jack Reed, D-R.I.

Jack Reed, the senior Senator from Rhode Island who just won re-election, is poised to become the top Democrat on the Armed Serves Committee in the next Congress.

In 2007, the West Point alumnus rose to national prominence when he delivered Democrats' response to then-President George W. Bush's State of the Union address, calling the war in Iraq "a flawed strategy that diverted attention and resources away from hunting down Osama bin Laden's terrorist network."

Prior to his election to the Senate in 1996, Reed served three terms in Rhode Island's State Senate and three terms in the U.S. House. After his graduation from West Point in 1971, he received an active duty commission in the Army, and earned a master's degree in public policy and later, a law degree from Harvard.

Though Reed's name has been floated as a potential replacement, Reed's spokesperson says the Senator "does not wish to be considered" for the Secretary of Defense or any other Cabinet position.

"Senator Reed loves his job and wants to continue serving the people of Rhode Island in the United States Senate," said Reed's Press Secretary, Chip Unruh. "He has made it very clear that he does not wish to be considered for Secretary of Defense or any other cabinet position. He just asked the people of Rhode Island to hire him for another six year term and plans on honoring that commitment."

ABC News' Jonathan Karl and Arlette Saenz contributed to this report.