David Sterling is facing off against Leslie Rutledge.
Rutledge received the most votes on during the May 20 Primary, but not enough to avoid a runoff.
The winner will face Democratic state Rep. Nate Steel and Libertarian nominee Aaron Cash.
The polls are open until 7:30 p.m.
Copyright 2015 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.