What did Donald Trump do today?

He gave Sayfullo Saipov, the man accused of the recent NYC terror attack, his best chance at avoiding a conviction leading to the death penalty.

Trump, who has already thrown a wrench into one criminal prosecution in the last few weeks, today made his own prosecutors' jobs much more difficult with this tweet about Saipov:

...There is also something appropriate about keeping him in the home of the horrible crime he committed. Should move fast. DEATH PENALTY!

— Donald J. Trump (@realDonaldTrump) November 2, 2017

This was the second time in as many days that Trump publicly demanded the death penalty for Saipov.

Based on the overwhelming evidence against Saipov, a conviction for murder or terrorism charges in state or federal court would normally be all but guaranteed--and while the federal death penalty is rarely employed, Saipov might be a prime candidate. (One of the last people to be executed by the federal government was Timothy McVeigh, who also used a rental truck to commit terrorist murder.)

But Trump's statements amount to a gift to Saipov's defense team, who will now be able to argue that Trump--speaking as president and in a forum that reaches tens of millions of potential jurors--has tainted the jury pool. This will likely hinder prosecutors' efforts to seat the jurors they'd most like, especially if they had intended to seek the death penalty.

In one respect, though, Trump's comments are a sort of improvement from his previous statements on the death penalty, in that Saipov is certainly responsible for the attack. Trump also called for the execution of the so-called Central Park Five, black and Latino men who were wrongly convicted in 1990 of a violent rape. They were later exonerated by DNA evidence and the confession of the actual rapist, and were paid $41 million by the city in a settlement that was influenced in part by Trump's campaign against them. Trump still claims to believe that they are guilty.

Why does this matter?

• A president who cannot control himself well enough to avoid sabotaging the work of the Justice Department is incompetent to hold office.