One source familiar with the process said Clinton is expected to let her supporters know who she picked sometime Friday, possibly before an afternoon event in Tampa, Fla. That announcement may come through a text message or email, the source said.

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The two candidates will likely campaign together tomorrow in Miami.

"I would bet all my chips on Kaine," one confidant of Clinton's said Thursday.

One longtime Clinton friend said the Pence pick frees Clinton up “to go with someone who can be president immediately and with whom she has a strong relationship with no requirements to satisfy new demographic or diversity consideration.”

Clinton said earlier this week in an interview with Charlie Rose that she was “afflicted with the responsibility gene” and wanted someone who would be experienced above all.