Senate Minority Leader Mitch McConnell, who faces a competitive re-election race next year, has been named to a watchdog organization's list of the most corrupt members of Congress.

The nonpartisan group, Citizens for Responsibility and Ethics in Washington, included McConnell in the annual list because of lingering questions over whether he used congressional staff to conduct campaign opposition research. That charge originates from a conversation among McConnell's campaign staff that was secretly recorded earlier this year.

"The recording indicated Sen. McConnell’s then-chief of staff and several legislative assistants helped compile opposition research on some of the senator’s potential campaign opponents while on the taxpayer’s clock," the group said.

McConnell's office has said any campaign work conducted by congressional staffers was done on weekends or at night, but CREW could not find evidence on McConnell's campaign payroll of the congressional staffers mentioned in the recording.

The group concludes that "if members of Sen. McConnell's congressional staff were engaged in campaign activities during office hours and using Senate resources, Sen. McConnell likely violated ... Senate ethics rules and federal law."

McConnell is running for re-election against Kentucky's Democratic Secretary of State Alison Lundergan Grimes. He also faces a Republican primary challenge from businessman Matt Bevin.