Obama and Clinton through the years

Joe Biden, Barack Obama and Hillary Clinton talk before a Democratic presidential debate.

Barack Obama talks to the crowd at a fish fry in Columbia, S.C.

Democratic presidential candidate Sen. Hillary Clinton, D-N.Y., answers a question alongside Sen. Barack Obama, D-Ill., during a Democratic presidential primary debate in Manchester, N.H.

Barack Obama shows Hillary Clinton renovations done to Chicago’s Soldier Field during a break in an AFL-CIO forum.

Barack Obama greets Hillary Clinton at the end of a Democratic presidential debate in Coral Gables, Fla.

Hillary Clinton and Barack Obama respond to a question during a Democratic presidential debate in Cleveland.

Barack Obama and Hillary Clinton speak aboard a plane on their way to join a rally in Unity, N.H.

Then-Sen. Barack Obama takes the stage with then-Sen. Hillary Rodham Clinton at a campaign event in Unity, N.H.

Then-Sen. Hillary Clinton, D-N.Y., listens as Democratic presidential candidate Sen. Barack Obama, D-Ill., speaks during a campaign stop in New York.

Then-President-elect Barack Obama after announcing that Hillary Clinton is his choice for secretary of state during a news conference in Chicago.

President Barack Obama tours the Sultan Hassan Mosque with then-Secretary of State Hillary Clinton in Cairo, Egypt.

President Barack Obama and then-Secretary of State Hillary Clinton make their way to the Rose Garden of the White House to discuss the fatal attack on the U.S. Consulate in Benghazi, Libya.

Then-Secretary of State Hillary Clinton, accompanied by President Barack Obama, speaks during a Transfer of Remains Ceremony at Andrews Air Force Base, marking the return of the remains of the four Americans killed in the attack on the U.S. Consulate in Benghazi, Libya.