Texas Election Code - ELEC § 221.009. Compelling Voter to Reveal Vote

(a) A voter who cast an illegal vote may be compelled, after the illegality has been established to the satisfaction of the tribunal hearing the contest, to disclose the name of the candidate for whom the voter voted or how the voter voted on a measure if the issue is relevant to the election contest.

(b) If the number of illegal votes is equal to or greater than the number of votes necessary to change the outcome of an election, the tribunal may declare the election void without attempting to determine how individual voters voted.