Possible, but not probable. Putin knows that if he oversteps the mark, he will have te whole of NATO to deal with. That is what NATO is there for.
As for nuclear. It would be a the case of the big turkey voting for Christmas. NATO and the US could blow Russia out of the waterin a very short period of time, if they started using nuclear weapons. remember Russia has a Pacific seaboard and so do the USA and Canada, also a NATO member, so he would have to be prepared to immediately fight on two very long fronts, one running from Finland to the Black Sea and the other stretching 4,500 kilometres from north of North Korea to the Berents sea - and Alaska nearly touches Russia. I am sure that the US will have well prepared air force and military bases there.