Inside Newt Gingrich's tax return

NEW YORK (CNNMoney) -- Presidential candidate Newt Gingrich was the first over the line in the Great Tax Return Race of 2012.

On Thursday night, just as the final debate before South Carolina's Republican primary was getting underway, Gingrich posted online the 2010 joint federal tax return he filed with his wife, Callista.

The headline number was 31%.

That's the percentage of the couple's total income -- $3,162,424 -- they owed in federal income taxes. Their total tax bill was $994,708.

Gingrich's opponent Mitt Romney said this week that he estimates his effective federal tax rate is about 15% -- a number driven down by Romney's millions in investment income, which is typically taxed at a lower rate than ordinary income such as a salary.

By contrast, the vast majority of Gingrich's income -- more than $2.5 million -- came from money he earned from his businesses, the biggest of which is Gingrich Holdings.

Most of that $2.5 million was levied at ordinary income tax rates, said Mark Luscombe, principal federal tax analyst at CCH, a tax information publisher.

And ordinary income tax rates run as high as 35%.

That's why Gingrich pays a larger percentage of his total income in taxes to Uncle Sam than Romney. While some of Gingrich's income was taxed at the lower rates of 10%, 15%, 25%, 28% and 33% -- everything over $373,650 was taxed at 35%.

George Romney's tax transparency

At the CNN debate Thursday, Romney again said he would release his 2011 tax return when it is completed in April, and added that he may also release returns from prior years as well.

In addition to his businesses, Gingrich drew income from speaking and board of director fees, a rental property, and from his many stocks, mutual funds and certificates of deposit.

Gingrich's return also showed that he paid close to $20,000 in alimony in 2010, nearly $15,000 for household help and more than $8,000 in tax preparation fees. He also received $76,000 in pensions and annuities.

He and his wife also recorded $81,133 in charitable deductions for the year, although it's mostly unclear which organizations they donated to. The only obvious one: The couple donated $9,540 to the Basilica of the National Shrine of the Immaculate Conception.

By the end of 2010, the couple had paid $613,517 in taxes. That meant when they filed their return for that year, they had to cut a big check to the IRS -- nearly $383,000 -- to pay the rest of their tax bill. And since they hadn't paid at least 90% of what they owed throughout the year, they also owed an underpayment penalty of $1,543.