GREELEY, Colo. -- Republican U.S. Rep. Marilyn Musgrave's re-election campaign was already heated, and it just got smelly as well: Her staff accused a Democratic activist Thursday of leaving an envelope full of dog feces at Musgrave's Greeley office.

Musgrave spokesman Shaun Kenney said someone stuffed the envelope through the mail slot in the door on May 31 and then sped away in a car. Kenney said most of the preprinted return address was blacked out, but staffers used the nine-digit ZIP code to trace it to Kathleen Ensz, a Weld County Democratic volunteer and a professor emeritus at the University of Northern Colorado.

Ensz told The Associated Press she left the envelope at Musgrave's office but said it "wasn't in the office doors, it was in the foyer." Asked what she meant by the act, she declined comment.

Kenney demanded an apology from Musgrave's likely Democratic opponent, state Rep. Angela Paccione of Fort Collins.

Paccione spokesman James Thompson denied the campaign had anything to do with it.

"We find that kind of act to be completely deplorable," he said. "We're not in the business of dirty tricks like that. This type of thing is really out of our control, but of course we'll do anything that we can to discourage this."

Thompson said Ensz, vice chairwoman of a state Senate district committee for the county Democratic Party, has no formal ties to the Paccione campaign.

Kenney said police were asked to investigate. Greeley Police Sgt. Joe Tymkowych said Ensz had been cited for criminal use of a noxious substance, a class-three misdemeanor. She is due in court Aug. 10.

Musgrave, who is seeking a third term, is the House sponsor of a constitutional ban on gay marriage. One GOP analyst has said she was among the party's most vulnerable lawmakers in this year's election.

Ensz began teaching at UNC in 1970 and earned her emeritus status in 2000, UNC spokeswoman Gloria Reynolds told the Denver Post.