Here's the analytical error that's causing Republicans to back Trump

In my column today on Donald Trump and the unacceptable tail risks he poses (such as possibly defaulting on US government debt), I quoted Marco Rubio because he seemed, back in February, to understand the tail-risk argument against Trump.

You don't give "the nuclear codes of the United States to an erratic individual," advised Rubio - not because Trump is probably going to employ those codes, but because the risk of him doing so is unacceptably high.

Now, Rubio is supporting Trump, even as he refuses to retract his past criticisms, including that quite devastating one about the nuclear codes. How is that possible?

David Frum explains the thinking that allows establishment Republicans to justify their tortured endorsements of Trump in his new Atlantic piece about how Trump has broken many of the protective guardrails in American politics.

"Once you've convinced yourself that a president of the other party is the very worst possible thing that could befall America," Frum writes, "then any nominee of your party - literally no matter who - becomes a lesser evil."

Many American politicians have gotten so used to demonizing candidates from the other party, talking about them as not merely people who will make bad policy but as people who will destroy the country, that they no longer have a vocabulary for talking about outcomes (like nuclear war) that are worse than being governed by the other party.

Indeed, Rubio now says he's "even more scared about" Clinton than Trump, and that he "can't live with" a Clinton presidency.

Of course, that's nonsense. Rubio already lived with a Clinton presidency, not to mention a Carter presidency and an Obama presidency. He'll live with another Clinton presidency just fine.

You can tell Rubio knows this on some level because, unlike with Trump, he doesn't talk about having unease about Clinton and the nuclear button. That's because, whatever her many flaws, there's no greater reason to fear Clinton will up and use nuclear weapons than there was with any recent president of either party. There is a good reason to fear this with Trump.

But the one thing you can't do in politics is admit there's an outcome worse than the other party winning, and Rubio knows it.

This rule puts Rubio and Republicans like him in the absurd position of acting like capital gains tax policy is more important than nuclear war - and of urging voters to take the risk of a global calamity rather than voting against their party - but hey, that's how it works.