In a statement released Thursday, Gibson says he is endorsing former state Rep. Paul Sadler for the Democratic nomination. Gibson said he is a strong supporter of organized labor and did not want to run without their support. The AFL-CIO endorsed Sadler on Wednesday.
Gibson said he would continue to support Democratic candidates in the 2012 election.
That leaves only two top-tier candidates left in the Democratic race to replace retiring Sen. Kay Bailey Hutchison. Sadler and party activist Sean Hubbard.
The primary election is scheduled for April 3, but the date remains uncertain due to a legal dispute over the state's political maps.