Texas constitution
Article 16: general provisions
Section 65

(b) If any of the officers named herein shall announce their candidacy, or shall in fact become a candidate, in any General, Special or Primary Election, for any office of profit or trust under the laws of this State or the United States other than the office then held, at any time when the unexpired term of the office then held shall exceed one year and 30 days, such announcement or such candidacy shall constitute an automatic resignation of the office then held, and the vacancy thereby created shall be filled pursuant to law in the same manner as other vacancies for such office are filled.