(Business Insider)
Michael Hayden.

Michael Hayden didn't mince words when asked about Donald Trump's proposal to bar Muslim immigrants and tourists from entering the US.

It's not legally possible, the former director of the Central Intelligence Agency told Business Insider in an interview. It's morally reprehensible.

And, frankly, "it's dumb as dirt."

"Why in God's name would you want to close the borders of our nation to the adherence to one of the world's great monotheisms?" Hayden said during an interview in Business Insider's Manhattan headquarters.

It was part of extensive comments critical of Trump, the presumptive Republican presidential nominee of whom many top Republican officials remain wary.

Hayden, the former director of the National Security Agency whom President George W. Bush nominated in 2006 to head the CIA, is one of a number of prominent past and present national-security officials who have remained skeptical of Trump's qualifications as commander in chief.

Hayden took aim at Trump over his plan to erect a wall along the US-Mexico border, his proposal to bar Muslims from entering the US, and his suggestion that the US might have to target the families of terrorists, among other things.

"He has not shown me the capacity to treat international issues with the complexity and seriousness that they deserve," Hayden said. "And again, if I'm supposed to ignore those things — 'that was just done for the crowd' — frankly, that's an even worse problem. I'm going to take him at face value."

Hayden's first two choices succumbed to Trump. He had endorsed former Florida Gov. Jeb Bush, swinging to Ohio Gov. John Kasich after Bush's ouster from the race.

Now? Out of the three major-party candidates left, Hayden says Hillary Clinton, the Democratic frontrunner, is best prepared in terms of national security.

"Of the candidates left, the only one who in any way seems to embrace the American liberal post-World War II foreign-policy consensus, have a fairly active American role in global things — the only one left standing is Hillary Clinton," Hayden said.

(Thomson Reuters)
Republican US presidential candidate and businessman Donald Trump

Hayden is hardly alone in the pantheon of national-security officials, especially those who have served in both Democratic and Republican administrations.

Former Secretary of Defense Robert Gates, who served under both Presidents George W. Bush and Barack Obama, told Business Insider in January that it was difficult for him to imagine a Trump administration. Though he didn't name Trump specifically, he criticized what he portrayed as candidates' fantastical proposals.