Clinton is scheduled to campaign in Florida on Friday and Saturday, but The Wall Street Journal -- citing people familiar with the former secretary of state's selection process -- said the news could come Friday in a text message to supporters.
The Washington Post said Clinton would likely introduce her pick either later today or at a Saturday rally in the Sunshine State. The Journal and the Post, citing unnamed sources, said Kaine has emerged as the frontrunner.
Kaine is seen as a leading contender for several reasons: he could help Clinton claim Virginia, a key battleground state and one her campaign believes is key to victory, and also could help cement her overwhelming lead with Hispanic voters.
The 58-year-old senator also has strong foreign policy experience -- he currently serves on the Senate Foreign Relations and Armed Services committees. And he is seen as helping Clinton win votes from independent male voters.
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Also on the short list are: Agriculture Secretary Tom Vilsack, who has been friends with the Clintons for decades and is from another key battleground state, Pennsylvania; and Labor Secretary Thomas Perez, a Latino seen as a true liberal.
Her Republican challenger Donald Trump, who accepted his party's nomination on Thursday with a speech painting a dark picture of an America in crisis, has chosen Indiana Governor Mike Pence as his running mate.
The Democratic National Convention, at which Clinton will be formally crowned the party's White House candidate, begins Monday in Philadelphia.