Virginia Sen. Tim Kaine is the leading choice for Democrat Hillary Clinton's running mate — and former President Bill Clinton has privately expressed his support for him, according to news reports."I would bet all my chips on Kaine," one of the former secretary of state's confidantes told The Hill on Thursday.Bill Clinton backs Kaine because of his strong domestic and national security credentials, three Democrats briefed on the conversations with the former president told The New York Times. Kaine, 58, was elected to Congress in 2012 and sits on the Senate Armed Services Committee. A former chairman of the Democratic National Committee, he also served as Virginia's governor and lieutenant governor — and as Richmond's mayor.His background, Bill Clinton believes, "appeals to voters and makes him prepared for the presidency," the sources told the Times.However, the former president, they said, is leaving the decision entirely to his wife. Hillary Clinton is expected to announce her decision as early as Friday and campaign with her choice on Saturday in Florida.Agriculture Secretary Tom Vilsack is another top contender — and others viewed as finalists include Ohio Sen. Sherrod Brown, New Jersey Sen. Cory Booker and Labor Secretary Tom Perez, according to both reports.Massachusetts Sen. Elizabeth Warren has also met with Clinton, though the presumptive nominee is expected to pick more of a "centrist candidate" in light of Republican Donald Trump's selection of Indiana Gov. Mike Pence, his vice presidential candidate, the Hill reports.A longtime Clinton friend told the newspaper that the Pence choice frees her 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."Bill Clinton has close relationships with Vilsack and Perez, according to the Times.In 2008, Barack Obama considered Kaine for his No. 2 slot before selecting Delaware Sen. Joe Biden, the Times reports.The White House signaled Wednesday that the president would support the former governor's presence on the Democratic ticket.A Clinton spokesman did not respond to requests for comment from both the Hill and the Times.