Wyoming - The State Senate is currently 2/30 Democrats, and both of those seats would probably vote Republican if merged with a nearby Republican one. So the answer is quite higher than 15...possibly something like 25.
The city of Laramie, seat of Albany County and the University of Wyoming, votes narrowly but consistently Democratic and seems to be about 1/18th of the whole state, so 18 could be enough (it was Clinton+4 but with loads of Gary Johnson votes that are a one-off circumstance, then Biden+13). I toyed around in DRA and you can do even better by pairing Teton County with the Wind River Indian Reservation, bringing it down to 14 if not 13, but I am pretty sure that constitutes gerrymandering.
