I don't think he'd nuke us, but I do think he could use a 'tactical' nuke in the Ukraine. He's ideologically and politically committed to winning or at least an end result that he can present as a win to his own population. The more they flounder in Ukraine, the more likely it becomes that he does something desperate. I personally think he'll settle for bits of Eastern Ukraine and the Crimea with a new iron curtain partitioning the country. A low yield nuke dropped on the other bit as an act of petulance would not surprise me in the least - almost like Saddam setting fire to the oil wells on his retreat from Kuwait.