The former president, whose wife lost a hard-fought primary to Obama, has not always had kind words for the Democratic nominee.
At one point, he compared Obama's experience level to his own in 1988, when he decided not to run, because "I really didn't think I knew enough, and had served enough and done enough to run." He called Obama's candidacy a "fairytale" and compared it to that of Jesse Jackson, which was seen as a slight by Obama loyalists.
But Clinton, who, as Politico notes, "moved slowly toward a full-throated endorsement this summer," has since offered kind words for his wife's former rival.
"Barack Obama is ready to lead America and restore American leadership in the world. Ready to preserve, protect, and defend the Constitution of the United States," Clinton said at the Democratic National Convention. "Barack Obama is ready to be President of the United States."
Details about the joint appearance on the stump – the first for the pair – have not yet emerged.
UPDATE: A Democratic source tells CBS News the joint appearance will be Wednesday night in Orlando, Florida, and that Hillary Clinton will not be present.