Mitt Romney finally released his tax returns today, and the big winner was President Barack Obama.

That’s because Romney and his wife paid only $6.2 million in taxes on $42.5 million in income in 2010 and 2011, an effective tax rate of less than 15 percent. That’s a lower tax rate than most Americans pay, including Warren Buffett’s secretary.

This is why the timing of Romney’s tax disclosure helps Obama. When the president gives his State of the Union speech tonight, Buffett’s secretary, Debbie Bosanek, will be sitting in first lady Michelle Obama’s box. That’s because Obama will provide further details on how to implement what he calls the “Buffett Rule”—a proposal he made in September to require that people who make more than $1 million a year pay at least the same effective tax rate as middle-class Americans.

The rule is named after Buffett because the billionaire investor has complained it isn’t fair that he pays a lower tax rate on his income than his secretary pays.

That’s because Buffett—and the Romneys—make most of their money from investments, which are subject to capital gains tax rates of 15 percent. If these gains were treated as ordinary income, Buffett and the Romneys would pay the top tax rate of 35 percent.

There’s a good reason to tax capital gains at a lower rate than ordinary income: to encourage people with capital to put it to work by investing in businesses. Even Obama believes this in principle, at least when it comes to investments in qualified small businesses, which the president wants to exempt from capital gains taxes.

Romney has proposed eliminating capital gains taxes for taxpayers who earn less than $200,000 a year. His main Republican opponent, Newt Gingrich, has gone further, calling for elimination of capital gains taxes altogether.

“Under that plan, I’d have paid no taxes in the last two years,” Romney pointed out during Monday night’s Republican debate in Tampa, Florida.

Most Americans probably would have a problem with that.

The attention that Romney has drawn to capital gains rates also could spell trouble for hedge fund managers, venture capitalists, and managers of other investment partnerships, including real estate firms. These managers pay the capital gains tax rate of 15 percent on most of their compensation—their share of their firms’ investment profits. Obama has called for taxing this compensation, known as carried interest, as ordinary income. This would discourage investment, opponents contend.

Romney released his 2010 tax return and estimates for 2011 in response to requests by his Republican opponents. Gingrich, meanwhile, responded Monday night to requests to release the contract he had with Freddie Mac after the former House speaker left Congress. He did so, but for only one year. The contract shows Gingrich’s firm, the Gingrich Group, received $25,000 a month from the government-sponsored enterprise in 2006.

Gingrich insists he did no lobbying for Freddie Mac, which along with Fannie Mae helped prop up the housing market during the housing bubble by buying mortgages from lenders, including lots of subprime loans. When these investments went bad, the federal government was forced to take over both companies.

But the contract sheds little light on what Gingrich did do for Freddie Mac—it just cites “consulting and related services.”

That’s not enough to make this issue go away.