Editor's note: This story has been updated with a comment from Gov. Rick Perry's office.
Duncan, R-Lubbock, left the Senate to become chancellor of the Texas Tech University System, a role he assumed this month. The special election will install a senator to serve the rest of Duncan’s term, which ends in 2017.
Lucy Nashed, a spokeswoman for Perry's office, said he chose to hold the election in September — rather than on Nov. 4, as some had expected — to ensure that the new senator could take office by the start of the next legislative session.
State Rep. Charles Perry, R-Lubbock, is among those who have declared their candidacies for the Senate District 28 race. His opponents include Republicans Jodey Arrington, a former Texas Tech vice chancellor, and Epifanio Garza, who ran against Duncan in 2012, as well as former Sweetwater Mayor Greg Wortham, an independent who has clashed with some Tea Party Republicans over the issue of wind energy.
All candidates seeking to run must file applications by Aug. 1, with early voting set to start Aug. 25.
As a special election, the race is open and nonpartisan, without a primary. It could be one of the fall’s most competitive, with Perry and Arrington having amassed significant war chests already.