Joaquin Castro endorses Hillary Clinton
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Rep. Joaquin Castro (D-Texas) endorsed Hillary ClintonHillary Diane Rodham ClintonGOP struggles with retirement wave Overnight Energy: Trump to revoke California's tailpipe waiver | Democrats propose bill to revoke Trump endangered species rollback | Trump officials finalize rule allowing fewer inspectors at pork plants Mark Mellman: The most important moment in history? MORE for president in an email sent by the Ready for Hillary group on Monday. 

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"There's no doubt about it: Hillary is the best person to be our 45th president," Castro wrote in the email."Hillary has always been a tireless advocate for working families -- she's never ceased to make sure everybody has a fair shot at achieving the American Dream."

Castro is seen as a rising star in the Democratic Party, and someone who could run for state-wide office down the road. 

In August, The Washington Post reported that Castro's brother, Julian, had dinner with Bill and Hillary Clinton, stoking speculation about a vice presidential nod. 

"She hasn't announced yet that she's running in 2016, but Hillary needs to know that if she does, millions of grassroots supporters like you will be standing proudly by her side," Joaquin Castro's email continues. "You and I both know Hillary would do amazing things as U.S. President -- but it's up to us to make these early moments count."

It ends with a fundraising request for Ready for Hillary, which is not directly affiliated with Clinton but has been laying the groundwork for a run and sending staffers to work in key elections this year. 

"Chip in $20.16 right now: say you'll stand with Hillary in 2016," the message states. "And don't forget, all contributions made before midnight tomorrow will be doubled."