Senator Tim Kaine, Democrat of Virginia and an early supporter of Barack Obama in the 2008 primaries, is expected to announce on Saturday that he is joining Ready for Hillary, a “super PAC” that is trying to build grass-roots support for a potential presidential run by Hillary Rodham Clinton in 2016.

Mr. Kaine is the latest of many Democrats who supported Mr. Obama against Mrs. Clinton in 2008 but who are now backing her in 2016. Last week, Caroline Kennedy, the American ambassador to Japan and the daughter of President John F. Kennedy, told ABC News that she would “absolutely” support Mrs. Clinton. Senator Claire McCaskill of Missouri has also declared her support for Ready for Hillary.

Mr. Kaine said that in the 2008 race, “I made my decision early because I knew something: He was the right person for the job, but getting there would be hard.”

“I’m stating my support for Hillary Clinton today for the same reason,” he said, according to prepared remarks released late Friday. He planned to formally announce his endorsement at a South Carolina Women’s Democratic Council breakfast in Columbia on Saturday.