Putin backed into a corner will surely pull the trigger on the nuclear warheads, if they work. If they work as good as all the other weapons they had in long term storage. Will we really do anything about it if he launches a small tactical nuke at a medium sized city like we did in Japan? Is that the red line that Biden won't let Russia cross? Just like the red line in Syria or Afganistan or Iran or wherever. We need seem to have the fortitude to the right thing. It's one thing to blow up a city in Ukraine with nuclear weapons and it's another thing to send an ICBM right at Washington DC.

BTW, if Trump gets elected, this is mostly a moot point as he will immediately pull all support for Ukraine and this will be mostly a forgone conclusion.