John Boehner is the latest top Republican to struggle to explain why he's supporting Donald Trump

On Thursday, John Boehner faced a question that's been on my mind: Given Donald Trump's apparent lack of interest in implementing a conservative policy agenda, why should Paul Ryan want him to be president?

Steven Rattner, the former "Car Czar" to President Barack Obama, asked Boehner that question at the SkyBridge Alternatives hedge fund conference in Las Vegas.

Importantly, this is a different question than whether Ryan should endorse Trump. Ryan might have any number of strategic and self-preservation reasons to do that - for example, he might think a unified Republican Party will lose many more seats in Congress than a divided one, or that failing to endorse Trump could cause him to lose his position as House speaker.

It's harder to say why Ryan should think a Trump presidency would be good, but if he makes an endorsement he's going to have to act like he does, even if he's only making the endorsement out of strategic necessity.

Boehner is a good person to talk to if you're trying to get inside Ryan's head, since they've known each other forever; Boehner noted Ryan put up yard signs for him in Boehner's first congressional campaign, 26 years ago.

But when asked why Ryan should be pleased about the prospect of a Trump presidency, Boehner paused for several seconds, seemingly stumped. Then he offered a not-very-satisfying answer that boiled down to Trump having won the nomination, fair and square.

"Listen, Donald Trump is the presumptive nominee whether people like it or not," he said. "Clearly, he has said a lot of things over the course of this campaign that some would be critical of, on the right and the left. So I think Paul was just being cautious. But I don't have any doubt that there's going to be a meeting of the minds and this is going to get smoothed over."

He added that Ryan might have the opportunity to help Trump change for the better.

"I think what Paul is trying to do is shape the direction of Trump's policies," he said. "At some point, he's going to have to lay out some policies on how he's going to make America great, how he's going to deal with immigration. At least having some principles they can agree on would be helpful."

But of course, Trump is not a totally empty vessel on policy. Rattner proceeded to walk Boehner through a list of the more controversial positions Trump has laid out, asking if Boehner agreed with them.

Does John Boehner favor a temporary ban on Muslims entering the US? "No."

A border wall paid for by Mexico? "No."

Ripping up trade agreements and slapping 35% tariffs on imports? "No."

Using torture measures worse than waterboarding and considering killing the relatives of terrorists? "Might be a bit harsh."

An isolationist, "America First" turn in foreign policy? "Not quite my style."

So given all that, Rattner asked how it's possible for Boehner to support Trump.

"Listen, I voted for John Kasich," Boehner said. "John Kasich didn't win. Jeb Bush didn't win. Thank God the guy from Texas didn't win."

This is a real problem for Republicans as they struggle to coalesce around a nominee so many of them have such deep reservations about. Saying you support Donald Trump is one thing. Sounding like you mean it is another.

"Listen, I voted for John Kasich" is not a pro-Trump argument that makes you sound like you mean it.