I'm not sure how this would work, Would they just drop nuclear tactical bombs in their own people currently holding onto what they have in territory?..Would they have to move them out then bomb?
Guess in his mind it would be justification to use such weapons on attackers, i.e. Ukranians in Ukraine.
Whole thing is bonkers of course but then again the war started as him sending troops as "peacekeepers" into those regions. Hasty manner in which all of this is done is an indication that he's really rattled by latest counter attack and must be feeling a lot of pressure at home.
Suppose now that whole thing has gone wrong for him, plan is to declare those regions Russian territory. Politically he can then say he got back what is allegedly historically Russian territory to try and save face with the public. From military point of view this allows him to go on the defensive with no further large attacks into Ukraine planned and hide behind nuclear weapons, should further counter attacks by Ukraine be planned.
He's now doing his best to try to build a narrative in Russia that they are in danger from NATO hence mobilisation and concentrate his resources at home to prevent any kind of revolution from happening.