A Washington, D.C., ethics watchdog has filed a complaint against Sen. John Ensign based on activities surrounding his extramarital affair with a former campaign aide.

Citizens for Responsibility and Ethics in Washington -- CREW -- filed a complaint Wednesday with the Senate Select Committee on Ethics and a complaint with the Federal Election Commission against Ensign's campaign committee and leadership PAC.

The group claims Ensign may have violated criminal law and certain ethic rules, including possible discrimination on the basis of sex and illegal contributions made to his PAC.

"It appears that Senator Ensign has abused his position as a member of congress and it's appropriate for the Senate Ethics Committee to investigate that," the group's executive director Melanie Sloan told FOXNews.com.

The Nevada senator confessed at a press conference in his home state last week to having an extramarital affair with an unnamed woman. The woman was later identified as 46-year-old Cindy Hampton, a former campaign treasurer for two committees connected to Ensign. Hampton's husband, Doug, had served as an administrative assistant on Ensign's Senate staff.

CREW alleges in its complaint that Ensign may have terminated the Hamptons from their staff positions for reasons related to the affair, which began in December 2007 and ended in August 2008.

"If true, the senator likely engaged in discrimination on the basis of sex in violation of Title VII, and Senate Rule 42, which incorporates the Civil Rights Act of 1964 to Senate employees and prohibits discrimination based on sex," the group said in a press release Wednesday.

Cindy Hampton may have received a severance payment from Ensign when she left his campaign committee and PAC, but neither committee has reported any such contribution. The CREW complaint alleges that Ensign may have violated criminal law if he failed to report a PAC contribution of more than $25,000.

Ensign spokesman Tory Mazzola has claimed Hampton's husband made "exorbitant demands for cash and other financial benefits" through an attorney within the past month -- a charge the Hamptons have denied.

Ensign, meanwhile, has quietly returned to work as questions continue to linger over the affair -- and his reason for publicly admitting to it. Ensign has said he intends to remain in the Senate, though he resigned as head of the Republican Policy Committee, the fourth-ranking spot in the leadership. He offered an apology to his GOP colleagues on Tuesday.

Click here to read the complaint filed with the Senate Ethics Committee.

Click here to read the complaint filed with the Federal Elections Committee.