Sadler won a unanimous endorsement from the political arm of the Texas AFL-CIO last weekend, and that sparked Gibson's announcement.
That leaves three Democrats, ten Republicans and a Libertarian in the race to succeed Republican U.S. Sen. Kay Bailey Hutchison, who isn't seeking reelection.
Here's his full statement:
After careful consideration, I have decided to not seek the Democratic nomination for United States Senate. The extensive time and energy needed to raise funds and build name ID around the state in a very narrow timeframe meant too little time and energy left for my family, my law practice and my duties as President of HTLA. This is the right decision for me and my family at this time.
This was my first time seeking elected office, and I knew that as an outsider, I’d be up against others with institutional advantages. My heart and my upbringing are with the labor movement and I did not want to move forward without their endorsement.
It has been an honor to travel and talk to voters all over Texas, and I truly appreciate the supporters who embraced our message of a new direction for our great state.
I still believe that Texas deserves a hardworking U.S. Senator who will challenge gridlock in Washington – that’s why I am endorsing fellow Democrat Paul Sadler.
I remain a proud Texas Democrat and I will explore how I can be helpful in getting other Democrats elected to office in 2012 and beyond.
Texas Tribune donors or members may be quoted or mentioned in our stories, or may be the subject of them. For a complete list of contributors, click here.