This data sounds like it could have been someone's copy of the voter file, the database political groups keep on every American who can vote. Versions of that file have leaked before. There's some speculation some of the leaks have been deliberate, to illegally avoid campaign finance laws.
Got a link? That sounds like interesting reading. (The explanation of how leaking a database of every voter's info helps exploit a loophole in campaign finance law, I mean, not the data.)
I don't have a link, but here's how that would work: there are strict limits (a few thousand per donor) on how much an official campaign can raise/spend. Outside/independent groups have no such restrictions (see Citizens United) - they could take a billion dollars from one donor and spend it all on behalf of one candidate. But what good would the official restrictions be if the campaigns could call the independents and say "buy a million dollars of ads for me". So there are also strict laws about "coordination". You can probably figure out the rest by yourself... But leaving the records in a place where they are "available to the public" provides plausible deniability to the coordination problem.
