Correct: Powell v. McCormack (1969)
It's good that there is a prior court ruling, but expect McConnell to try to get a new ruling overturning the 1969 one. At least he could use it to delay letting Moore take his seat.
I think Moore saying that McConnell shouldn't be the Senate majority leader led McConnell to use the nuclear option...we will probably find out he instigated the Washington Compost hit piece.