evidence: The Biden campaign did not respond to requests for comment. Spokespeople for Clinton and Obama declined to comment. [...] The overarching goal of a fundraiser where Biden, Clinton and Obama share a stage is to raise a significant amount of money, two of the people familiar with the discussions said. The expectation is the event would bring in both big and small donations. [...] Still, both Clinton and Obama remain popular among Democrats. And they are more popular Democratic leaders than Biden, though former presidents generally tend to enjoy higher approval from Americans than the current one.