The problem is that it’s simply not credible on its face. No matter what is said, there is no possibility of US leaders launching nukes over Ukraine.

It’s a bad idea to bluff in situations like this, because when the obvious bluff is called and you are forced to back down, it makes enemies more likely to test all your commitments and “red lines”.

While Putin might not be willing to risk MAD either over Ukraine, it’s a lot more believable that he would than that the US would. There have been direct kinetic attacks on Moscow and parts of Russia have been invaded. There have been assassination attempts against people close to him. This is not some far away conflict for him. He’s already taking on a huge amount of risk, a lot more than the US has taken on in a long time—even in WW2 there was no direct attack on the continental US.