James Mattis Has Emerged as a Favored Contender for Defense Secretary

— -- Retired Marine Corps Gen. James "Mad Dog" Mattis, has emerged as a favored contender for nomination as secretary of defense in the Trump administration, ABC News has learned, winning strong public praise from the president-elect and an endorsement from retired Gen. Jack Keane.

Keane, who met with Trump earlier this week at Trump Tower, told ABC News that he was offered the job as secretary of defense but withdrew his name from consideration because of other commitments and personal reasons. (He lost his wife earlier this year).

Trump transition officials would not confirm whether an offer was made but indicated the president-elect had been very interested in Keane, even as he expressed reluctance to accept a cabinet appointment.

Keane told ABC that at the meeting last week he recommended Mattis and retired Gen. David Petraeus to the president-elect as candidates for the defense secretary job.

Trump met with Mattis on Saturday at Trump's estate in Bedminster, New Jersey, after which Trump said the retired general, who formerly headed the U.S. Central Command, was "the real deal."

Today, Trump tweeted that Mattis "is being considered for secretary of defense."

Federal law requires that defense secretary appointees are at least seven years removed from active duty, so an appointment of Mattis would require a waiver from Congress, if he were nominated and confirmed.

Such an action is not without precedent.

Gen. George C. Marshall, who served as Army chief of staff and as a member of the Joint Chiefs of Staff during World War II, needed a waiver to serve as secretary of defense under President Harry Truman. He is believed to be the last defense secretary who needed a waiver.