Depends on what they think about it, which may not be what we think they should think about it.I do not think that will ever happen. Even in Russia, there is a chain of command that has to follow out such an order and all of them know it will be the last breath they take.
As a thought experiment, imagine the US had fallen apart and in the process Texas became its own nation. In the process of attempting to aggressively re-build the US "empire", Russia (imagine the Soviets had won the Cold War here) armed the Texans with all kinds of high tech stuff to defend itself....and the Texans were about to massacre a large US force.
If a US POTUS were to say at that time: "Back the fuck off Russia, or I will use tactical nukes!", would it be wise for the Russians to assume with utter confidence that he's bluffing and'or US generals would surely not obey him?
As Michael Koffman put it a couple week ago: "Look deep into the beady eyes of General Gerasimov for a few moments and see if you can see anything in them that leads you to believe he's too principled to obey that order."