Democrats opposed to President Bush's nomination of Sam Fox as the new U.S. ambassador to Belgium posed a new challenge Thursday, charging that the fact Fox will serve without pay makes him a 'volunteer,' and that the government cannot accept his unpaid services.

Democrats are vehemently against the appointment and are investigating a new twist in the former Swiftboat Veterans for Truth contributor's appointment.

It is possible Fox might now be, as a "volunteer", in violation of another U.S. law, Democrats charge, in his new position as an ambassador.

In a letter sent today, Senate Foreign Relations Committee Democrats Chris Dodd, John Kerry and Bob Casey ask the General Accounting Office, the nonpartisan watchdog arm of Congress, to expedite an investigation into a conflict between competing U.S. codes that could, if resolved as Democrats hope, leave Fox without a job.

In a Democratic memo obtained by FOX News that made the rounds on Capitol Hill last week when the nomination was pulled by the White House, an analysis of GAO recess appointment research shows a possible conflict in Fox working for free and the government being unable to accept unpaid services from a person in a position, like ambassador, that has a fixed salary in federal statute.

In some instances, political appointees can serve without a salary if they agree in advance they have no future claim to one, but U.S. law also seems to say that those with salaries fixed in law (ambassadors have a fixed rate of pay set in the Foreign Service Act of 1980) cannot serve voluntarily, according to the Democrats' analysis.

The memo states, "The voluntary services prohibition does apply, says the GAO, where the position in question has a fixed rate of pay. In such a case, the voluntary services prohibition in Title 31 applies (on the theory that because the salary is a statutory entitlement, it cannot be waived)."

Democratic aides in the memo conclude by asking, "So, how to reconcile this clear conflict...? That is the $64,000 question."

Sam Fox contributed $50,000 in 2004 to Swiftboat Veterans for Truth, a group that challenged the Vietnam credentials of Democratic presidential candidate John Kerry.