Texas Election Code - ELEC § 146.025. Filing Period

(a) Except as otherwise provided by this code, a declaration of write-in candidacy:

(b) If a candidate whose name is to appear on the general election ballot dies or is declared ineligible after the third day before the date of the filing deadline prescribed by Subsection (a), a declaration of write-in candidacy for the office sought by the deceased or ineligible candidate may be filed not later than 5 p.m. of the 75th day before election day.

(c) A declaration of write-in candidacy filed by mail is considered to be filed at the time of its receipt by the appropriate authority.