If I were them and had to choose one or the other — and it is essentially one or the other, as the Susan Collins crowd is unlikely to call both — I’d choose Bolton.
But I can see the argument for Mulvaney.
In any case, Chris Murphy understands that Democrats don’t really have a choice here. Bolton is gettable. Mulvaney is not, at least not without the sort of court battle that the Collins crew just won’t countenance.