ABC News' Michael Falcone and Shushannah Walshe report:

Presidential candidate Rick Santorum, who frequently touts his humble roots as the grandson of a coal miner, earned an average of more than $900,000 each year between 2007 and 2010, according to the candidate's tax returns released on Wednesday.

His adjusted gross income varied from about $659,000 in 2007 to more than $1.1 million in 2009. In 2010, Santorum earned $923,411.

According to the IRS-1040 forms released by the Santorum campaign, the former Pennsylvania senator paid an effective tax rate of roughly 28.5 percent in 2010.

That is a substantially higher rate than what Mitt Romney paid the same year, according to the former Massachusetts governor's tax returns, which his campaign released in January.

In 2010, Romney made $21.7 million and paid nearly $3 million in taxes - an effective tax rate of 13.9 percent. Romney paid the lower rate because most of his earnings came from investments rather than wages.

"He had dividend income," Santorum said of his rival in an interview with reporters in North Dakota on Wednesday night. "He had capital gain sincome which is taxed at 15 percent. I had ordinary income which got taxed at a higher rate."

Santorum's campaign released four years of returns on Wednesday, but sources close to the candidate said they do not plan to call on Romney to do the same. The disclosure comes at a time when both he and Romney are making a particularly strong appeal to Michigan's working-class voters ahead of the state's Feb. 28 primary.

Santorum paid $167,759 in federal income taxes in 2007, $262,082 in 2008, $276,523 in 2009 and $263,442 in 2010. His income was highest in 2009.

"I went out after having served in the Senate and had to go out and make money and hopefully with the hope of trying to be able to build some assets," Santorum said. "It turned out that most of the assets I ended up building was paying down my mortgage on a house that went down in value."

The returns show that Santorum and his wife, Karen, who are the parents of seven children, donated a smaller portion of their income to charity than the Romneys in 2010. That year, the Santorum's made more than $16,000 in charitable contributions compared to more than $3 million for the Romneys.

Newt Gingrich, who released his 2010 tax returns in January, paid an effective tax rate of 31 percent on the $3.1 million he made.

All four years of Santorum's tax returns indicate they were "self prepared." The tax forms were first posted by Politico and their authenticity was confirmed to ABC News by the Santorum campaign.