Republican presidential candidate Rick Santorum, whose humble coal-country biography is central to his political message, made more than $3.6 million in recent years as a Washington consultant and claimed a German luxury sedan as a business expense, according to tax returns released by his campaign.

Santorum’s growing wealth after leaving Congress could complicate his attempt to focus on his history as the grandson of a hardscrabble coal miner, particularly as the GOP nomination race narrows to a contest between him and former Massachusetts governor Mitt Romney, a former private-equity manager.

The new financial details came as the Santorum and the Romney campaigns converged in their newest battleground of Michigan, trading brickbats over who was most opposed to the auto industry bailout and labor unions.

Romney also effectively killed a March 1 debate in Georgia by deciding to back out, leaving just one such event between now and Super Tuesday on March 6.

Meanwhile, former House speaker Newt Gingrich, who is attempting to stage a third comeback in the race, capped a fundraising tour in California, where the campaign said it had raised $2 million so far this year.

Santorum has sought to play down the money he made as a Washington consultant-for-hire after his time in the Senate, focusing instead on his working-class background and socially conservative beliefs. The strategy helped produce a spate of primary victories and a rapid rise in the polls, leaving him as the main rival to Romney in Michigan and other upcoming contests.

But the tax returns released by his campaign could serve to remind voters of his strong ties to Washington, where he and his wife earned as much as $1.1 million a year after he left Congress in 2007. Much of Santorum’s income was earned from contracts with firms such as Pennsylvania-based Consol Energy, though Santorum never registered as a lobbyist, disclosure records show.

He paid federal income taxes ranging from $167,000 in 2007 to $310,000 in 2009. Santorum said during a CNN interview that he felt “very blessed” in his financial success after leaving Congress, but said he and his wife also had a “couple setbacks” that forced them to take out a series of mortgages on their house.

“I felt very successful in . . . you know, making money,” Santorum said during the Wednesday night appearance, adding: “Also was very successful in paying taxes.”

Personal tax rates have become a flashpoint in the presidential race largely because of Romney, a multimillionaire former equity fund manager who pays about half the tax rate of his rivals because most of his income comes from investments. President Obama and other Democrats have proposed a higher rate for such income, an idea generally opposed by Republicans.

Santorum paid an effective federal tax rate of about 28 percent from 2007 to 2011, well above the 13.9 percent rate paid by Romney in 2010, according to returns released by the two candidates.

Gingrich paid about 32 percent of his $3.2 million income in taxes in 2010, according to his federal returns. Rep. Ron Paul (R-Tex.) is the only major presidential candidate who has not released his returns, joking in one debate that he would be embarrassed by his lack of wealth.

Romney, who has reported in disclosure forms that he is worth as much as $260 million, has struggled to connect with lower-income voters in the early GOP nominating contests, a gap that Santorum is hoping to exploit. In an appearance at the Detroit Economic Club Thursday, Santorum characterized himself as “a candidate who cares about everyone and gives them the opportunity to rise in society.”

Santorum also attacked Romney for supporting the government loans given to Wall Street banks during the economic crisis, arguing that he is more conservative than Romney by opposing all bailouts.

“Governor Romney supported the bailout of Wall Street and decided not to support the bailout of Detroit,” Santorum said. “My feeling was that government should not be involved in bailouts, period. I think that’s a much more consistent position.”

Romney spokeswoman Andrea Saul called Santorum a “typical politician” who had said favorable things about the Wall Street bailout in the past. “Senator Santorum went to Washington and never left — in word and in deed,” Saul said.

Santorum, 53, has had a rocky history when it comes to residency issues, coming under fire during the 2006 campaign for living outside of Pennsylvania and for receiving a mortgage from a private bank run by a major campaign donor. The father of seven also clashed with a Pennsylvania school board for enrolling several of his children in online classes through that district despite living in Virginia.

The newly released tax returns, which were first provided to Politico Wednesday night, show Santorum and his wife claiming mortgage deductions for their McLean house, which sits on a five-acre lot with a swimming pool, according to county records.

The house was initially bought by a trust formed by Santorum and a McLean real estate attorney, carrying a mortgage of $2 million — more than the property’s peak assessed value of $1.65 million in 2008, Fairfax County records show. The house was transferred in 2009 to Santorum and his wife, who hold a $1 million mortgage on the property, which is now valued at about $1.25 million, records show.

Santorum also owns five rental properties in State College, Pa., and claimed business-related deductions for a Chevy Trailblazer in 2007 and an Audi A6 in 2008.

Santorum and his wife, Karen, trail their rivals by giving about 1.8 percent of their income to charity in 2010, when they earned $923,000. Gingrich and his wife were next, with a 2.8 percent charitable giving rate.

Romney and his spouse donated about 13.8 percent of their $21.6 million in gross income to the Mormon church and other charitable causes in 2010. President Obama and first lady Michelle Obama reported donating 14.2 percent of their 2010 income to charity.

Helderman reported from Detroit. Staff writers Amy Gardner in Los Angeles and researcher Lucy Shackelford in Washington also contributed to this report.