The War on Tactical Nuclear Weapons

Biden has repeatedly declared various weapons system off limits to send to Ukraine because of their escalatory dangers. The latest reversal comes in his announcement that he will now set send F-16 fighter jets, we will now send them to Ukraine as they aggressively expand their military operations inside of Russia. If we give F-16s, why not give them the green light to use them to bomb targets inside Russia if we haven't already? To bomb the Kremlin with our F- 16s? Or why not give us tactical nuclear weapons? What's the argument against that? Are we willing to risk a third world war over the question of who governs the Donbas region of Eastern Ukraine filled with Russian