America's constitutional system of checks and balances relies, as the ultimate check, on the simple human emotion of shame.

Unfortunately, President Trump is shameless.

The president once again demonstrated his shamelessness — naturally — in a Monday morning tweet.

Actually, there's plenty of legal debate over whether the president has the power to pardon himself. But even if he does: Wouldn't that take an incredible amount of chutzpah?

Yes. But that's not a quality Donald Trump lacks, is it?

The president's inability to feel — or, at least, his ability to completely disregard — shame has long been established. He's the man who mocked John McCain's time as a prisoner of war and the disability of a New York Times reporter, and who once mused about his hypothetical ability to shoot someone on Fifth Avenue without losing any electoral supporters. And that's not to mention his treatment of women. Words and acts that would drive other men to red-faced apologies instead are defended endlessly by this president and his supporters. The list of examples goes on and on: If nothing else, Trump's willingness to plaster his name on real estate on multiple continents has long demonstrated a certain lack of humility, which is a precondition for shame.

But what does shame have to do with the Constitution?

Plenty, as it turns out. A glance at the Federalist Papers tells us two important things about the president's pardon power: First, that it was expected the power would be used to correct injustices — including cases where the law had fallen too hard upon someone who was properly convicted. Second: The Founders expected the president's temptation to misuse his pardon power would be tempered by the fact that, well, he wouldn't want to look like he was misusing his pardon power.

"The reflection that the fate of a fellow-creature depended on his sole fiat, would naturally inspire scrupulousness and caution," Alexander Hamilton wrote in Federalist 74. "The dread of being accused of weakness or connivance, would beget equal circumspection, though of a different kind."

In other words: A responsible president wouldn't want the public shame of granting an unjust pardon.

Hamilton, known for a bit of shamelessness himself, probably never imagined a president like Trump. Connivance is plainly part of the package; circumspection less obviously so.

Trump, through his pardons, has already demonstrated whom he sees as his "fellow creatures" worthy of mercy — people like Sheriff Joe Arpaio and conservative provocateur Dinesh D'Souza, men prominent for their ugly treatment of immigrants and minorities, and popular with Trump's base for those stances. (They've also been done without the "scrupulousness and caution" that Hamilton expected — Trump granted both pardons while bypassing the presidential office responsible for evaluating petitions for clemency.) Yes, he also pardoned Jack Johnson, the turn-of-the-20th-century boxer who was the target of racist law enforcement, but that may have been the exception that proves the rule: Trump has not offered such compassion to any living black man.

There is one explicit limit on the president's pardon powers: He cannot save anyone — including himself — from impeachment.

We don't know what Robert Mueller's investigation will determine: Right now, we have more speculation than evidence about what's going on inside the special counsel's offices. At this point, however, it's clear that if he finds wrongdoing by Trump or his inner circle, Trump will issue a pardon in an attempt to short-circuit justice. Rather than feeling shame, it's almost certain he would defy any judgment against him with pride.

That would put the ball squarely in Congress' court. And there's little evidence the Republican-held Congress would be inclined to impeach a chief executive from their own party. And if Democrats take Capitol Hill in November, they would have to mount any impeachment by themselves, without bipartisan cover.

Which means the problem of shame isn't limited to Trump. As long as the Republican Party continues to turn a blind eye to the president's lies and misdeeds, shamelessness will continue to rule the land.