The mistake the Tennessee republicans made was letting Corker get through the primary. Corker just looks lost and gives the impression of being tired and old. It doesn't help matters that Ford is young, very well spoken and slick.
Someone like Corker is the absolute last person you want running against someone like Ford.
Still, I think that Corker might pull it out. It just depends on how motivated Tennessee republicans are as midterm elections are all about party turnout. In presidential election years you have to worry about convincing the undecideds but those people don't even vote in the midterms.
The Florida republicans letting Harris get through the primaries was another huge mistake. Those are two senate seats that were easy wins that the GOP primary voters threw away.
Nevertheless, compared to Ford, Corker's the Second Coming.