Rep. John J. Duncan, Jr. has known both of the two front runners to replace him for both of their careers.
WATE reports here that Duncan is endorsing Jimmy Matlock in the August 2 Republican Primary.
I would suspect part of his reason for not endorsing the Knox County Mayor is all the speculation that has surrounded the Mayor in particular during his last term of office and the RUMORS of a TN Comptroller investigation and RUMORS of the FBI looking into some of his behaviors.