Just as he is presenting himself to voters in two Midwestern states as the son of a coal-mining family with strong ties to blue-collar America, Republican hopeful Rick Santorum has released four years of tax returns that show earnings hovering around $1 million (£633,000) a year since leaving Congress in 2006.

While the wealth of Mr Santorum, an ex-Senator from Pennsylvania, seems paltry beside that of Mitt Romney, whose front-runner status in the GOP nomination contest he is trying to usurp, it does not tally with the pick-up truck narrative that has helped him connect with religious conservatives. Among the more surprising details in the filing: Fox News has paid him almost $250,000 as a commentator.

A poll last night from the Detroit News showed Mr Santorum leading Mr Romney by 34 per cent to 30 per cent in Michigan, which holds a primary election on 28 February. While it may be a statistical dead heat, the numbers will only pile new pressure on Mr Romney, for whom Michigan was meant to be a firewall. He was born in the state and his father, George Romney, served as governor there. A loss there next week to Mr Santorum could be devastating.

The Santorum tax filings could be helpful to the Romney camp as it tries to brand Mr Santorum both as a Washington insider and member of the grubby "K Street" lobbying club in the capital. While not registered as a lobbyist, Mr Santorum has drawn a large portion of his earnings in consulting fees for corporations, notably in the energy and medical sectors, which benefited from his support while he was a Senator.

Mr Santorum, who campaigned yesterday in Michigan and Ohio, which will be one of 11 states voting on Super Tuesday in two weeks, likes to stress his western Pennsylvania roots. But since he lost re-election to the Senate in 2006, he has settled in a four-bedroom house in Virginia close to Washington on five acres, worth $1.4m in 2010.

Still serving as a senator in 2006, Mr Santorum earned around $165,200 annually. But his financial picture improved sharply upon leaving and setting up his consulting company, Excelsior LLC. According to his filings he earned $659,000 in 2007, $952,000 in 2008, $1.1 million in 2009, before dropping slightly to $923,000 in 2010.

Still, Mr Romney will have to direct his fire carefully, not least because Mr Santorum earns 4 per cent of what he takes in each year. The filings also show the Santorum family paying double the tax rate applied to Mr Romney, whose effective 13.9 per cent tax rate is a sore point with many voters. Most of Mr Romney's income comes from investments – hence the lower bracket – while Mr Santorum has actually worked for his.

How much? Candidate earnings

Mitt Romney

Earnings in 2010 $21.6m

Tax rate 13.9%

How? The former Massachusetts governor has three homes and lives mainly on income from investments built during his years at the private equity powerhouse Bain Capital.

Ron Paul

Net worth Between $2.4m and $5.4m

Tax rate Unknown. He says he has no intention of publishing his tax return.

How? The 76-year-old physician turned congressman turned presidential candidate reports an annual income of $91,185 from a medical pension, and has an investment portfolio stuffed full of mining stocks.

Newt Gingrich

Earnings in 2010 $3.1m

Tax rate 31%

How? The former House speaker has built a network of businesses since leaving office 12 years ago. Most of his wealth comes from Gingrich Holdings, an umbrella company for a collection of lucrative enterprises.

Rick Santorum

Earnings in 2010 $923,000

Tax rate 31%