Craig is hoping to avoid repaying $217,000 in campaign funds the Federal Election Commission claims he misused to defend himself.
The FEC sued Craig in June in U.S. District Court in Washington, D.C., alleging he converted the campaign money to personal use by spending it on his legal defense after he was accused of soliciting sex in a Minneapolis-St. Paul International Airport bathroom. The commission argues Craig's defense had no connection to his campaign for federal office.
Craig counters that money tied to his airport bathroom trip was neither for personal use or his campaign, but falls under his official, reimbursable duties as senator because he was traveling between Idaho and the nation's capital for work.
He cites a U.S. Senate rule in which reimbursable per diem expenses include all charges for meals, lodging, hotel fans, cleaning, pressing of clothing - and bathrooms.
"Not only was the trip itself constitutionally required, but Senate rules sanction reimbursement for any cost relating to a senator's use of a bathroom while on official travel," wrote Andrew Herman, Craig's lawyer in Washington, D.C., in documents filed Thursday.
In its complaint, the FEC contends the three-term U.S. senator's campaign account, Craig for U.S. Senate, paid at least $139,952 to the law firm Sutherland, Asbill and Brennan in Washington, D.C., and $77,032 to Kelly & Jacobson in Minnesota for legal services related to his guilty plea to disorderly conduct.
An undercover officer said Craig tapped his feet and signaled under a stall divider that he wanted sex.
Regulators who voted unanimously in May to pursue the complaint against Craig after informal negotiations failed are seeking repayment of the money, as well as fines of up to $6,500 from the former senator and his treasurer, Kaye O'Riordan.
Contacted Friday, FEC spokeswoman Judith Ingram cited pending litigation and declined to comment on the case.
Craig also didn't return a phone call Friday to his lobbying firm, New West Strategies.
In documents supporting his bid to have the complaint dismissed, Craig cites the case of former U.S. Rep. Jim Kolbe of Arizona, who tapped campaign money in 2006 to defend himself after allegations of improper behavior emerged against him following a Grand Canyon rafting trip with two former male pages.
The trip by Kolbe, the second openly gay Republican to serve in Congress, was an official visit with support provided by the National Park Service.
The FEC concluded that Kolbe's use of the campaign money to pay legal expenses associated with a Department of Justice inquiry regarding the trip were "ordinary and necessary expenses incurred in connection with his duty as a House member."
Craig's lawyer argues that his airport bathroom visit, made while traveling back to Washington, D.C., from his home state, should be seen similarly - and the FEC complaint dismissed with prejudice.
"Simply put, no principled distinction can be drawn between the Kolbe matter and this case," Herman contends. "Sen. Craig's legal expenses arose during official Senate travel, an activity that was part of his constitutionally enumerated duties as a holder of federal office."