Thompson garnered about 44 percent of the statewide vote in his 2008 run for railroad commissioner. He lost to Michael L. Williams, who received about 4 million votes to Thompson's 3.4 million.
Former ambassador Tom Schieffer and author and humorist Kinky Friedman have said they are considering running for the Democratic nomination.
On the Republican side, Gov Rick Perry has said he will run for re-election. He will be challenged by Sen. Kay Bailey Hutchison in the primary election.
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