The 20% of Americans who owe more on their mortgages than their home is worth — the gateway to foreclosure — probably don’t know Edward J. DeMarco by name, but they’re familiar with his work. As acting director of the Federal Housing Finance Agency, DeMarco oversees Fannie Mae and Freddie Mac, which collectively control about half of U.S. home loans. Something else worth knowing about Mr. DeMarco is that Democrats and some advocacy groups want him fired. Demarco has the power to help distressed homeowners by writing off principle balances on their mortgages, but won’t because it doesn’t look good on the books. The Treasury Department recently tripled the incentives it pays for principal reduction under the administration’s anti-foreclosure program, which has DeMarco running a new costs and benefits analysis. Depending on what he comes up with, this could give him at least a few more weeks at his post.