The former San Antonio mayor told The Associated Press he’ll announce a decision next month.
The move gives the 44-year-old an early start to what’s expected to be a crowded Democratic field without a clear front-runner to challenge President Donald Trump.
Castro was on the short list of running mates for Hillary Clinton in 2016.
Talk of his 2020 candidacy has been eclipsed so far by bigger Texas names, including Democratic Congressman Beto O’Rourke, who lost his Senate bid to Republican Ted Cruz last month.
An exploratory committee lets potential candidates raise money and is usually a steppingstone to a formal campaign launch.
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