"Secretary Hillary Clinton is the best person to be our 45th President for many reasons," Kaine said. "She is a classic American optimist with the background and experiences necessary to lead this country in a very complicated world."
Kaine is joining Ready for Hillary, a pro-Clinton super PAC assembling a grassroots network in preparation for a Clinton campaign, becoming the second senator to do so after Sen. Claire McCaskill (Mo.). Like McCaskill, Kaine was among the most vocal Obama supporters throughout the hard-fought 2008 Democratic primaries.
Kaine joins a coterie of Obama donors, surrogates and advisers lining up behind Clinton in the run-up to 2016, including former senior Obama campaign officials Jim Messina, Jeremy Bird and Mitch Stewart. Last year, all 16 Democratic women senators signed a letter to Clinton encouraging her to run for president.
In February 2007, Kaine, then the governor of Virginia, became the first statewide official outside of Illinois to endorse then-Sen. Barack Obama's nascent campaign against Clinton, the establishment favorite.
In an interview Saturday morning, Kaine said he hopes fellow Democratic leaders rally around Clinton early.
"I sure hope a lot of folks will make plain to her that as she's trying to decide, 'Hey, if you run, we're all behind you,' " Kaine said. "I really hope people do that. But I don't have any illusion that it'll just be a one-candidate race. Competition isn't bad, and there might be some competition."
Asked whether he thinks other potential candidates -- including two with whom he has a working relationship, Vice President Biden and Maryland Gov. Martin O'Malley -- should take a pass on 2016 to clear the Democratic field for Clinton, Kaine said he "wouldn't presume to give them advice."
Kaine told the South Carolina Women's Democratic Council breakfast in Columbia that he recognized early in the 2008 cycle that Obama was "the right person for the job." Now, he believes Clinton is.
Kaine cited Clinton's "deep history of engagement" in such domestic policy issues as well as her "powerful background in international and foreign policy." He said she has "unmatched knowledge of the world, of our allies and opponents, and of global leaders -- public and private -- who can be partners for progress in the years ahead."
Kaine, 56, who represents a critical general election battleground state, is frequently touted by Clinton insiders as a potential vice presidential running-mate. In 2008, Obama's vice presidential search team vetted Kaine, but Obama instead selected the more seasoned Joseph R. Biden, a 36-year veteran of the Senate.
Before his election to statewide office, Kaine practiced law in Richmond and served on the Richmond City Council and as the city's mayor. He is fluent in Spanish, having spent a year during law school as a Catholic missionary in Honduras.
In the interview, Kaine said his vice presidential prospects did not influence his decision to back Clinton early. "It's nothing that's part of my thinking," he said. He added that in January 2017, he envisions himself sitting on the platform outside the U.S. Capitol for Clinton's swearing-in beginning his fifth year of service in the Senate.
Kaine said his endorsement on Saturday should be "a surprise" to Hillary and former president Bill Clinton. He said he did not discuss his decision with the Clintons, adding that he last saw the couple in January at Virginia Gov. Terry McAuliffe's inauguration.
Last October, Kaine and Bill Clinton shared a campaign stage in Richmond to stump for McAuliffe in his successful gubernatorial campaign. Clinton praised Kaine's tenure as mayor, noting in particular his work fighting crime. "I remember it very well," Clinton said.