U.S. Republican presidential candidate Rick Santorum has released his tax returns for the past four years, showing that he has earned more than $3.5 million since leaving the Senate, largely through ties to Washington.

Santorum, who has surged in recent public opinion polls, released the documents late Wednesday.

The documents indicate that since he lost his U.S. Senate seat in 2006, Santorum's income from public appearances, consulting work, and sale of stocks has increased from about $659,000 the first year out of office to around a million dollars a year now.

Santorum, the grandson of a coal miner, uses his humble roots to cast himself as a Washington outsider to set himself apart from his wealthier opponents Mitt Romney and Newt Gingrich. But the tax records reveal not only his wealth but also the fact that ties to Washington generate much of his income.

Release of tax records gives voters a chance to examine candidates' activities and financial ties through the sources of their income.

Taxes have also become a prominent issue in the Republican race because of controversy over a lower tax rate that applies to income made through investments rather than salary or wages.

Romney, whose 2010 tax return shows he earned $21 million that year, paid less than 15 percent of his income in taxes. Gingrich, who earned $3 million in 2010, and Santorum, who earned $930,000, each paid more than 25 percent.

The Obama administration has been calling for a change to that law, which mainly benefits the very wealthy, saying it is unfair to the middle and lower classes.