Hillary Clinton will receive a full-throated endorsement for president Saturday from Sen. Tim Kaine, D-Va., in South Carolina, a key early primary state.

Clinton, who has acknowledged she is weighing whether to run for president, has not yet declared her intention. But Kaine, speaking in Columbia on Saturday, will nevertheless offer his stamp of approval, calling Clinton a "classic American optimist with the background and experiences necessary to lead this country in a very complicated world."

"Our country will have no shortage of challenges to tackle in the future. ... And the very list of challenges on the table tells us who the next president should be," Kaine is expected to say Saturday at a South Carolina Women’s Democratic Council breakfast. "That’s why I stand with you today to encourage Hillary Clinton to run, to pledge my support for her candidacy if she does and to ask you to make that same pledge."

The early, prominent show of support for Clinton could be politically important to Kaine, who did not back Clinton in her last bid for president. When he was Virginia's governor, Kaine officially endorsed then-Sen. Barack Obama for president in February 2007, but told Obama privately much sooner, in 2006, that he intended to back him over Clinton.

"I made my decision early because I knew something — he was the right person for the job, but getting there would be hard," Kaine plans to say. "And I figured that the sooner I started helping him in Virginia, the more helpful I would be."

"I’m stating my support for Hillary Clinton today for the same reason," he will add. "She’s the right person for the job."

Kaine also will urge support for Ready for Hillary, the super PAC that has been working to build a nationwide network of Clinton's supporters.

Kaine was in South Carolina to keynote the annual South Carolina Jefferson Jackson Dinner, a major event on the Democratic Party's calendar in the state that usually draws presidential hopefuls.

In November, Sen. Chuck Schumer, D-N.Y., used a similar speech at an Iowa Democratic dinner to endorse Clinton and urge her to run.