It doesn't make sense to use them and then lose all support from that massive herd they've befriended over the last few years. Even if it's a tactical nuke, the very next minute the Chinese, Koreans and Indians will say they don't condone such behavior and in fact don't know who those people are, so please don't get us involved, etc.
The Chinese were also warning Putin to stop the pro-nuke charade, according to the NYT.