Trump is afraid Moore will lose and his Senate margin will decline. It’s already razor thin. Trump is used to New York politics where candidates must be lukewarm to be elected as a R. At this point it doesn’t matter, the R’s aren’t doing anything anyway, regardless of their margin.
In electing Moore the Republicans are more likely to have another Rand Paul. Moore will vote his conscience. Given his campaign rhetoric I would have expected him to be a no vote on Graham-Cassidy for the same reason why Paul voted no - it isn't a repeal.
There are not 20 Trump supporters in the Senate - and some of them, under certain circumstances, are Democrats.