I think that Putin will use a tactical nuke if he believes that the West doesn't have the resolve to respond in kind and will back down instead. The best defense is demonstrating that resolve and unity so Putin believes that using a nuke will result in a proportionate response. The biggest risk is showing weakness, a lack of unity, and/or a belief that Putin wouldn't actually do it.

In short, he will do it if he thinks he can get away with it, so we need to be extremely clear that he will not get away with it, and prepared to follow through if the worst happens.