Pressure on Mitt Romney to release his tax returns has reached a critical point as a stampede of Republican voices are now joining the call.

So far Romney has released just his 2010 return and an estimate of his 2011 return.

As the negative attention over the issue grows, Pema Leva wonders at TPM, "if it’s worth the bad press to keep the tax returns private, they must contain something worse."

While Romney's submitted returns show "an effective 2010 tax rate of 13.9 percent," previous years may be even more damning, making the bad press worth it.  Kevin Drum writes that "there are probably multiple years in which Romney paid no taxes at all."

Steve Benen speculates on other reasons for Romney taking the heat rather than releasing the returns, suggesting Romney "may have had to pay fines" for skirting tax laws or "may have additional offshore investments."

When asked by the conservative National Review about not releasing his returns, Romney stated: 

My tax returns that have already been released number into the hundreds of pages. And we will be releasing tax returns for the most current year as soon as those are prepared. They will also number in the hundreds of pages. In the political environment that exists today, the opposition research of the Obama campaign is looking for anything they can use to distract from the failure of the president to reignite our economy. And I’m simply not enthusiastic about giving them hundreds or thousands of more pages to pick through, distort, and lie about.

Calls for Romney to release his tax returns are now also coming from fellow Republicans.

Asked by reporters on Tuesday whether Romney should release more of his return, Texas Gov. Rick Perry said, “I’m a big believer that no matter who you are, or what office you’re running for, you should be as transparent as you can be with your tax returns and other aspects of your life so that people have the appropriate ability to judge your background and what have you."

The National Journal has a list of 14 prominent Republicans, including Perry, who also say Romney should release his returns. Their list includes Sen. Richard Lugar of Indiana, who said, "I have no idea on why he has restricted the number to this point."

The National Review Online also published an editorial Tuesday urging Romney to release his returns.