Biden shows how to take the high road against Trump — while also slamming him for his 'love letters' with Kim Jong-un

President Donald Trump has is already delighting in attacking his potential 2020 opponents, and he seems particularly focused on targeting former Vice President Joe Biden, who is currently leading the Democratic primary polls.

Many were appalled over the weekend, though, when Trump lashed out at Biden by claiming that North Korean leader Kim Jong-un, a murderous autocrat, had called the former vice president "low IQ" and "worse." Trump said he "smiled" at the insults. The idea that the American president was getting a kick out of insulting a leading Democrat alongside someone as atrocious as Kim was appalling, even by the low standards Trump sets for himself.

In addition to the plain, grotesque fact of the matter, the incident violated what was once thought of as a valuable norm in the United States — that politics stops at the water's edge. In other words, whatever disputes American politicians have domestically, they're ideally not supposed to engage these fights in the international context and instead present a unified front to the world.

Biden's team appropriately embraced this principle by holding off on responding to Trump's broadside until he had returned from his trip abroad. Biden spokesperson Kate Bedingfield then issued the following statement on Tuesday:

The President’s comments are beneath the dignity of the office. To be on foreign soil, on Memorial Day, and to side repeatedly with a murderous dictator against a fellow American and former Vice President speaks for itself. And it’s part of a pattern of embracing autocrats at the expense of our institutions

whether taking Putin’s word at face value in Helsinki or exchanging ‘love letters’ with Kim Jong Un.

Ed O'Keefe, a CBS News correspondent, noted that the statement came just "after Air Force One touches down at Andrews Air Force Base" and that it stuck to the principle that "politics stops at the water’s edge and you don’t criticize the president when he’s overseas."

It was a shrewd move. By adhering strictly to this principle and also responding just as soon as it no longer applied, Biden's team demonstrated two important qualities. First, it won't let Trump tear down the important norms that they believe are still valuable in American politics, and it will call the president out for flouting them. But second, that adhering to these norms doesn't mean Biden will be entering a fight with one arm tied behind his back — he'll respond forcefully and aggressively, when appropriate. This is a smart approach to take to avoid accusations that either that you're sinking to Trump's level or that you're bringing a knife to a gun fight.

The point about exchanging "love letters" with Kim Jong-un might have been over-the-top, except for the fact that Trump himself has spoken about the "beautiful letters" he's received from Kim and said they "fell in love." Trump's own actions and words make him difficult to parody and mean that it's almost impossible for someone to go overboard when criticizing him.