He is setting the scene so that he can claim NK are the aggressor and that he is just making the hard choice and doing what needs to be done. He will claim he warned them, but they didn't listen and that there was a genuine threat to the US and the world if he didn't do it. If NK keep chucking missiles over Japan then this is not going to be a hard sell for him.
The GOP might hate it, but I just can't really see them suddenly getting a back bone and shouting him down once he has actually dropped the bomb. By then it will be too late. They will grit their teeth and rally round, his supporters will love it and any criticism will be traitorous.
If he drops the nuke it will cement his dictatorship and we will all be fucked.
Sorry! 