But some in the GOP are backing Romney's secrecy.
"Let's say Mitt Romney released 100 years of tax returns tonight," said Alex Castellanos, a Republican media consultant who advised Romney in 2008. "What do you think the odds are that the Obama campaign would say, 'Oh great Mitt. Thank you! Now we can put that behind us and move on to more substantive issues like entitlement reform!' Zero."
Added Castellanos: "I'd advise Mitt to release 10 years of tax returns when Obama releases 10 years of birth certificates."
Castellanos liked this line so much, he put it on Twitter.
There are a couple of angles to this to keep in mind. The first is that really are legitimate questions that voters deserve answers to, and those answers are only available in the documents Romney is inexplicably keeping secret. No one is characterizing this as some kind of procedural hurdle that must be cleared; it's about resolving lingering, relevant questions about Romney's background.
Indeed, Romney publicly gave his word, on camera, that he would "go back and look" to let us know what tax rates he paid over the last decade, and it now appears the candidate will break that commitment without explanation.
Second, I'm not sure if Castellanos understands this, but when someone is born, they get one birth certificate. Since humans are only born once, and we don't get new birth certificates every year on our birthday, there's no such thing as "10 years of birth certificates."