Who: The Obama-Biden campaign.
What's going on: As long as Mitt Romney refuses to release more than two years of tax returns, he can expect to see ads like this one. Here's the punchline: "It makes you wonder if some years he paid any taxes at all." The fact is it makes you wonder all sorts of things, which we will all continue to do until he gives us reason to stop wondering.
Drip, drip, drip...