Does an elected official have to resign to run for a new office?
It depends. If an official announces their candidacy for a new office—which would include filing in the primary election---while still having a year and 30 days remaining in the term, the announcement is an automatic resignation under Article 16, Section 65 of the Texas Constitution. If an official announces their candidacy by filing when there is less than a year and 30 days left in the term, they do not automatically resign their office and may serve out the remainder of the term. December 2, 2019 is the year and 30 days point, while the primary election filing deadline is December 9, 2019 – so an official would have to file after the 2nd and on or before the 9th of December.