Texas Code of Criminal Procedure - CRIM P Art. 2.08. Disqualified

(a) District and county attorneys shall not be of counsel adversely to the State in any case, in any court, nor shall they, after they cease to be such officers, be of counsel adversely to the State in any case in which they have been of counsel for the State.

(b) A judge of a court in which a district or county attorney represents the State shall declare the district or county attorney disqualified for purposes of Article 2.07 on a showing that the attorney is the subject of a criminal investigation by a law enforcement agency if that investigation is based on credible evidence of criminal misconduct for an offense that is within the attorney's authority to prosecute. A disqualification under this subsection applies only to the attorney's access to the criminal investigation pending against the attorney and to any prosecution of a criminal charge resulting from that investigation.