In 2008, when John McCain was vetting Mitt Romney for vice president, Romney turned over 23 years of tax returns. McCain went on to pick Sarah Palin. This year, the American people are vetting Romney for president, and Romney has turned over one full year of his taxes.

Republican presidential candidate Mitt Romney. (Rich Schultz/AP)

But even if this one year is a fluke, even if it was done just for show, the outline of the story is clear enough: Romney is a very rich man who pays a very low marginal tax rate. In the past, it’s possible that more agressive tax engineering drove his rate even lower. It’s possible that more use was made of loopholes, and offshore accounts, and tax shelters. But even without all that, we know that Romney paid an effective tax rate of 13.9 percent in 2010. That’s likely the bulk of what Romney would have preferred to hide, and the bulk of what the Obama campaign needs to know.

As Greg Sargent writes, Romney, at this point, “personally symbolizes virtually the entire Democratic message about 2012 — he is the walking embodiment of everything Dems allege is wrong with our system and the ways it’s rigged in favor of the wealthy and against the middle class.”

His taxes also contextualize what he would do as president. Romney’s tax return shows that the effective rates paid by the richest Americans are often pretty low. Romney’s tax proposals would reduce those rates further. And the cost of those proposals -- more than $6 trillion when all is said and done -- would be offset through cuts to domestic programs. Romney is saying, in effect, that people like him should pay less to finance the government, and low-income Americans and seniors -- the two groups that depend on government programs -- should be asked to cut back instead. Many Americans are likely to see that as unfair.

That said, Romney’s taxes can be seen another way, too. The rich are job creators. They make money, they invest that money productively, and their investments create jobs. Democrats might prefer that the rich hand their fortunes over to the government, but that’s not, most Republicans would argue, how economies grow. During Romney’s career, he has made an enormous amount of money by taking over weak companies and, in many cases helping to turn them around, and he has used that money to finance further productive investments in the economy. That Democrats see something wrong with them is simply further proof of how little they understand about job creation.

For months, I’ve argued that the 2012 election, which most people think will be about jobs, will actually be about taxes. First, because a big tax cut is at the core of Mitt Romney’s policies for how to create jobs and a big tax increase on the top two percent is at the core of Obama’s thinking on how to reduce inequality. Second, because the GOP’s top priority is tax cuts and the top priority among liberals right now is raising taxes on the rich. Third, because Republicans think they have a winning issue in painting Obama as a tax hiker and Democrats think they have a winning issue in painting Romney as the second coming of George W. Bush 2.0. And now there is a fourth: Because Mitt Romney pays a lower tax rate than his secretary, and his tax plan would lower his tax rate even further, and Democrats will never, ever stop talking about that, and Republicans will never, ever stop defending it.