RELATED: What you need to know about Tim Kaine
In the message, Clinton says: "I'm thrilled to tell you this first: I've chosen Sen. Tim Kaine as my running mate."
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Kaine is expected to join Clinton at a rally in Florida on Saturday.
A favorite since the start of Clinton's search for a running mate, Kaine is a former governor of the battleground state and former mayor of Richmond.
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Kaine won election to the Senate four years ago after serving as the chairman of the Democratic National Committee.
Clinton is choosing Kaine from a group that included Agriculture Secretary Tom Vilsack and Massachusetts Sen. Elizabeth Warren.