Mitt, you need to disclose your tax returns, as should all of the candidates. You've already blown it by revealing your effective tax rate, so unless you're concerned you overstated your tax rate somehow, get off your seat and turn them over.
It's not merely because everyone wants to confirm you're a bajillionaire. It's because reviewing a tax return gives us an idea of where your money comes from and in what businesses or interests you're invested.
Saying that releasing them, maybe, in April is what you will do because it will divide Republicans is not a legitimate excuse. Moreover, by making such claims, you're underscoring the likelihood that people will be upset.
If this is the case, you create the impression that there's a smoking gun or something you don't want anyone to find out.
These aren't gotcha questions, game playing, or double standards. This is straight-up Presidential Elections 101. You won't win the nomination or an election if you pretend to be the Populist. You're not. That's ok, though. You just need to be the guy with the right ideas and skills. Tax returns are a valid measure of that.