Bob Gates thinks highly of Hillary Clinton - will he endorse her?

When former Defense Secretary Bob Gates left the Pentagon in 2012 after serving under two presidents, he remarked on his close working relationship with the secretaries of state that served during his tenure.

"For much of my professional life, the Secretaries of State and Defense were barely speaking to one another," he said. "In the case of Secretaries [Condolleezza] Rice and [Hillary] Clinton, I've not only been on speaking terms with these two formidable women, we've also become cherished colleagues and good friends."

Regarding Clinton, in particular, Gates has been largely complimentary. The two sat for not one but two joint interviews on "Face the Nation" during their time in President Obama's administration. In those sit-downs, they spoke of their broad agreement on the administration's policy priorities - an unusual harmony, to say the least, given the historic enmity between their cabinet departments.

After he left the administration, Gates continued to praise Clinton. "I think she was a good secretary of state," he told "Face the Nation" in 2015. "I think she played a critical role in getting the much tougher sanctions on Iran...We agreed in terms of the Afghan surge. If anything, she was tougher than I was on that...I think that we certainly agreed in terms of how to deal with the very first phases of the Arab Spring."

His memoir, "Duty," was even more effusive. "I found her smart, idealistic but pragmatic, tough-minded, indefatigable, funny, a very valuable colleague, and a superb representative of the United States all over the world," Gates wrote of Clinton.

To be fair, Gates has also leveled some criticism at Clinton as well. He acknowledged in 2015 that she was far more supportive of the 2011 Libyan intervention than he was. He wrote in his book about his disappointment after Clinton reportedly acknowledged in his presence that politics played a role in her decision to oppose the 2006 troop surge in Iraq. And he's said that Clinton's decision to use a private email server as secretary of state is "a concern."

Still, the sum total of Gates' remarks on Clinton shows that he clearly thinks highly of her. Asked on CBS Sunday Morning in 2014 whether he believes Clinton would make a good president, he replied, "Actually, I think she would."

So why hasn't he endorsed her 2016 presidential bid?

"I haven't been particularly impressed, frankly, by anybody at this point. On either side of the aisle," he said last May.

Asked whether he could see himself voting for Clinton, he said it was "a little early" to entertain the question, adding, "I'm not sure that having a Republican endorse you is the best thing at this point."

Since then, however, the landscape of the 2016 election has dramatically shifted. Clinton is on the verge of clinching the delegates necessary to win the Democratic nomination. She isn't as worried about her left flank as she was last year, so an endorsement from a Republican like Gates would probably do more good than harm at this point.

Perhaps more critically, Donald Trump has emerged as the presumptive Republican nominee, and Gates has expressed deep concern about Trump in recent months. He told Business Insider earlier this year that it's "difficult" for him to imagine Trump as president, criticizing the businessman's call for a ban on Muslims entering the U.S. and his proposal to build a wall along the U.S.-Mexico border.

"How he intends to deal with issues is pretty unrealistic," Gates said.

Will Gates weigh in more definitively on the 2016 election, given the recent clarification of the choice voters will likely face in November? Tune into our interview with him Sunday on "Face the Nation." Check your local listings for airtimes.