One of the richest Members of Congress is on the short list to be Mitt Romney’s running mate.

So is one of the poorest.

Of the four lawmakers the presumptive GOP presidential nominee is said to be considering, Sen. Rob Portman is by far the wealthiest, recently filed personal financial disclosure forms show.

The Ohio Republican, who served as director of the Office of Management and Budget and U.S. trade representative under former President George W. Bush, had a minimum net worth of $6.72 million at the end of 2011, up $1.4 million from the previous year.

The annual disclosures provide detailed information about lawmakers’ finances, including stock holdings, financial transactions and loans, but are only a rough guide to lawmakers’ actual wealth because assets and liabilities are reported in broad ranges. Roll Call subtracts the total minimum value of all liabilities from the sum of the minimum value of all assets to arrive at a minimum net worth.

Real estate holdings account for much of Portman’s wealth. The Cincinnati native owns about $1.2 million in commercial real estate, including the Golden Lamb, a historic hotel and restaurant in Lebanon, Ohio, worth at least $1 million. Portman made at least $415,000 from rent and capital gains on six commercial properties in Ohio, Kentucky and West Virginia last year, including at least $250,000 from the sale of a property in Nitro, W.Va.

Portman, who developed a strong political following over seven terms representing parts of southern Ohio in the House, has previously appeared on Roll Call’s annual ranking of the 50 richest Members of Congress and could very well make a comeback in 2012. In 2009 his minimum net worth was about $6 million.

On the other hand, Sen. Marco Rubio, a vice presidential hopeful, is likely among the least wealthy Members of the Senate with a net worth of negative $400,000.

The Florida freshman owns at least $127,000 in rental property, retirement funds and other assets, but those assets are offset by at least $100,000 in student loans, at least $350,000 in mortgages on his personal home, and a mortgage of at least $100,000 on a Tallahassee rental property, which he purchased in 2005 while serving in the state Legislature.

A spokesman for Rubio noted that the Senator purchased both properties at the height of the real estate boom and that the values of both have declined substantially since then. (Rubio’s neighbor in West Miami lost his home to foreclosure, the spokesman added.) In 2011, the city of Miami assessed his personal home at just more than $384,000.

“While he’s among the least wealthy Senators, it’s not as if he has financial problems,” said the spokesman, Alex Conant. “His personal finances just have more in common with the people he serves than the people he serves with.”

Such considerations didn’t prevent President Barack Obama from picking then-Sen. Joseph Biden (D-Del.), who in 2008 reported a net worth of negative $106,000, according to Roll Call’s calculations. Obama’s most recent personal financial disclosure forms, released in May, show assets of at least $2 million. Biden reported a minimum net worth of just $64,000 with assets of at least $239,000 and at least $175,000 in liabilities. Both men reported home mortgages exceeding $500,000.

Romney is also said to be considering several Republican governors, including Gov. Chris Christie of New Jersey and Bobby Jindal of Louisiana. The Romney campaign did not return Roll Call’s request for comment.

Rep. Paul Ryan (R-Wis.), the chairman of the House Budget Committee whose credibility with the conservative base makes him another possible vice presidential pick, has a minimum net worth of $1.9 million. Ryan holds a routine diversified portfolio, including modest positions in Apple, Altria, MasterCard, Amazon and Wells Fargo and tens of other firms. His only reported debt is a mortgage on his personal home of at least $250,000.

Sen. John Thune (R-S.D.), an early Romney backer who campaigned for him in Iowa and is also said to be on the short list, has a minimum net worth of $57,000. His assets of at least $157,000 are offset by at least $100,000 in home mortgages.

Lawmakers must report year-end values of bank accounts, stocks and rental properties but not of personal residences and federal retirement savings accounts. A lawmaker’s debts, including student loans, outstanding legal bills, credit card balances and mortgages on personal residences, are reported if they exceed $10,000 at any time during the year, even if they are paid off before the year ends.

A provision in the recently passed Stop Trading on Congressional Knowledge Act requiring lawmakers to report mortgages on personal residences as liabilities — but not as assets — will likely cause the reported minimum net worth of many lawmakers to dip in this year’s batch of disclosure reports, even if they have repaid other debts.

Rubio, for example, had a minimum net worth of $224,000 in 2010, but thanks to the new requirement now appears to be in much greater debt.