In a more or less evenly split district, a 6.8 victory would be quite respectable. And considered comfortable.
But this isn’t an evenly split district. It’s a very red district. Usually the Republican wins by thirty points or more. In fact, until this special election, the lowest the margin of victory for the GOP has been for the past generation is a little over 22 points.
Consider also that the Democrat didn’t have much help from his party (because the district was very red, and considered to be a lost cause), and the Republican wound up getting a lot of outside help when the race started to look close.
If I were a Republican, I would pay attention to these signs. And I’d worry about a 6.8 margin of victory in a place where a 30-point margin is expected. Because the Democrats are paying attention, and they’re emboldened.