What did Donald Trump do today?

He belatedly tried to de-incriminate himself over the Comey firing.

Tweeting this morning from his Mar-a-Lago resort home, where he is spending the whole week, Trump said that "Slippery James Comey, the worst FBI Director in history, was not fired because of the phony Russia investigation."

Trump's letter firing Comey did indeed cite other reasons--specifically, that he had been unfair to Hillary Clinton. Trump himself has constantly threatened to put Clinton in jail for unspecified crimes.

But almost immediately afterwards, Trump told NBC's Lester Holt that he was going to fire Comey no matter what, because of the Russia investigation.

Regardless of recommendation I was going to fire Comey ...Knowing, there was no good time to do it. And in fact when I decided to just do it, I said to myself, I said you know, this Russia thing with Trump and Russia is a made up story, it's an excuse by the Democrats for having lost an election that they should have won.

And the reason they should have won it is the electoral college is almost impossible for a Republican to win. Very hard. Because you start off at such a disadvantage. So everybody was thinking, they should have won the election. This was an excuse for having lost an election.

In saying so, Trump was all but confessing to obstruction of justice: it is against the law for an officeholder to fire an investigator to protect himself. This was one of the reasons Deputy Attorney General Rod Rosenstein then immediately appointed a special counsel to investigate the Russia matter directly.

Broadly speaking, the legal system tends to treat self-incriminating statements more seriously than it does self-exculpatory ones, on the grounds that a person carelessly admitting to a crime is more likely to be telling the truth than someone carefully denying it.

Why is this a problem?

• You can't un-confess to a crime just by contradicting statements you made of your own free will.