Well, it would be easier in an open seat. Moore should be facing a tougher race this year no matter the status of the gubernatorial race.
Two years ago Kris Kobach, the conservative candidate, won a very bitter primary. Moore beat him in the general election by 10 points. Two years before that Adam Taff, the moderate Republican, won a very bitter primary. Moore beat him by 3 points. Two years before that it was conservative Phill Kline who won a bitter primary. He lost to Moore by 3 points. See the pattern? So long as the Republicans are divided a Democrat has a chance.