Congress included the ban in its 2008 Department of Defense authorization bill, which Bush signed in January. The president called the ban unconstitutional in a signing statement, effectively declaring he would ignore the provision.

“Provisions of the act ... purport to impose requirements that could inhibit the president's ability to carry out his constitutional obligations,” Bush said. “The executive branch shall construe such provisions in a manner consistent with the constitutional authority of the President.”

In writing to Mukasey, Lee and her co-signers also express doubts about the agreement Bush signed with Iraqi Prime Minister Nouri al-Maliki in November. That agreement pledges U.S. support for Iraq’s security against both internal and external threats.

Some have expressed concerns that the deal is an effective treaty, one that circumvented Congress’s ratification authority.

“In light of such contradictions,” the letter reads, “we would like to learn precisely what is being done to make certain that permanent military bases are not being planned or constructed in Iraq and how the prohibition in law is being observed and overseen by the Bush administration.”

Defense Secretary Robert Gates sought to allay concerns about the deal earlier this week, telling Congress it would extend legal authority for the U.S. to operate in Iraq. Gates appeared at Senate and House Armed Services committee hearings Wednesday to discuss Bush’s Defense budget request for 2009.