CHARLOTTE, N.C. -- If there were millionaires at Tuesday's happy hour for prospective donors of the three major super PACs backing congressional Democrats and President Obama, they were not the recognizable backers of the groups. And there was not much exclusive about the affair.

At the door of a cozy, downtown bar a short walk from the site of the Democratic Party’s convention, an employee checked off names of attendees on an iPad. But he did not seem to stop anyone, and so this reporter walked on through the door. 

The event was one of several being hosted ...