Sen. Jim DeMint (R-S.C.) wants the National Labor Relations Board to turn over all documents related to its recent complaint against The Boeing Co. The NLRB, already facing a backlash for its meddling in a private company’s business decision, is now under fire for the secretive process it used to reach that conclusion.

DeMint’s wide-ranging Freedom of Information Act request suggests that politically connected special interests influenced the NLRB’s complaint against the company. The federal agency has asked an administrative law judge to halt expansion of Boeing’s operations in South Carolina in favor of heavily unionized Washington state. South Carolina is one of 22 right-to-work states.

DeMint is seeking documents that date to the beginning of the Obama administration, as well as any communications with the International Association of Machinists, the union at the center of the NLRB’s complaint.

“The public facts surrounding the complaint raise serious questions about the interpretation of the National Labor Relations Act upon which it is based, to say nothing of the troubling appearance of partisan, special interest politics at its heart,” DeMint wrote in a letter to the NLRB’s acting general counsel and FOIA officer.

In the letter, DeMint references a recent issue of the newsletter published by the International Association of Machinists Local 751, which boasts that “money gains your Union access to officials, which is critical to get our issues addressed and ensure our input is heard.”

DeMint questions if that constitutes “a pay-to-play system of political influence” that gave the union special access to NLRB officials. The union gave $1.9 million to Democrats during the 2010 election cycle.