The 2010 returns, which showed Romney paid $3 million in taxes on $22 million in income, have already given his opponents plenty of fodder for attacks. In a new ad airing in swing states, the Obama campaign notes, "Mitt Romney made $20 million in 2010 but paid only 14 percent in taxes -- probably less than you." For 2011, we already know the toplines: Romney estimated that he will pay $3.2 million in taxes on $21 million in income, for an effective rate of about 15 percent.

But the true devil was in the details of Romney's 2010 return. Deep in the thicket of Schedule This and Form That lurked evidence of his Swiss bank account and his holdings in the Cayman Islands and Bermuda. Vanity Fair detailed the evidence in an exhaustive story that painted Romney as "willing to push into gray areas" financially, and Democrats used it to paint Romney as unpatriotic -- offshoring his money just like he supposedly outsourced American jobs as a businessman.

It's stuff like this -- perfectly legal, but far outside the realm of most Americans' experience, and easy to portray as shady -- that has made Romney reluctant to disclose more. He's repeatedly said that releasing more returns would only serve to give the other side ammunition, and that's certainly true, even if you also believe it's information voters deserve to have to evaluate his candidacy. (A CNN poll this week found 63 percent believe Romney should release more tax returns.)

The tax returns play into the campaign's central theme, said GOP strategist Dan Schnur. "On the one hand, voters don't think Obama can fix the economy. On the other hand, they don't trust Romney to look after people like [them]," he said. "The taxes are a red herring, but they give the Democrats an opportunity to reinforce that perception of Romney."

Now, after weeks of pounding on the tax issue, it seems to be fading a bit as Romney has steadfastly refused to give any ground. "Romney has made a decision, and his decision on it is final. People have factored it into how they're going to vote," Steve Schmidt, the chief strategist for John McCain's 2008 campaign, told me. "In a country beset by problems, we are not going to spend 65 days in the fall talking about tax returns." Absent any new material to attack, Schmidt predicted the number of people obsessed with Romney's taxes would fade to a small, partisan hard core, like the Republicans obsessed with Obama's college transcripts.

The problem for Romney, though, is that by promising to release his 2011 taxes before the election while also putting off doing so, he has guaranteed that the tax issue will flare up anew -- and at a time when far more voters are paying attention to the campaign. Politically, it's puzzling that his campaign hasn't managed to get this out of the way already. It's possible that the 2011 return really isn't ready yet -- Romney might still be waiting on some paperwork from things like partnership or corporate income -- but you'd think he would have the best accountants money can buy. There's a clear advantage to dumping this information during the dog days of summer.

Instead, Romney seems bound to drop the new tax information at the most inopportune time. "They're going to have to do what they said they're going to do -- they can't get to the end of the campaign and just forget," Schmidt said. "It will be the dominant story for a day in the fall campaign."

And, if Democrats have their way, probably more.

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