A Madison man pleaded guilty Tuesday to making a telephone threat to blow up the offices of the Democratic Party of Wisconsin earlier this year, one of more than 100 calls he had been charged with making to the office.

William O. Diederich, 61, told U.S. District Judge William Conley in court that he doesn't have a clear recollection of what he said in the call on Feb. 24. Prosecutors said Diederich left a voice mail message saying that when the building explodes, the bombings of Sterling Hall on the UW-Madison campus and the federal building in Oklahoma City would "seem like a firecracker compared to what's gonna happen to you people."

Diederich, who was soft-spoken and polite in court, told Conley that he made the phone calls, but since suffering a fall that has kept him from working as an electrical contractor, "my memory is not too good at remembering what I said."

Assistant U.S. Attorney Jeff Anderson said that Diederich asked to be allowed to send a letter of apology to the Democratic Party, which Conley granted as the only exception to an order that otherwise bars Diederich from having any contact with the group.

Diederich now faces up to 10 years in prison and a $250,000 fine, though under federal sentencing guidelines he is all but certain not to receive a sentence that severe. Conley scheduled sentencing for Dec. 20.

In an affidavit filed in May by FBI agent Joseph Lavelle, Diederich was identified as the person who repeatedly made threatening calls and left threatening messages at the party's office at 110 King St. in January, February and March.

In the Feb. 24 message Diederich warned that the building was going to blow up "one of these days."

Lavelle's affidavit states that in other calls Diederich made threats to shoot anyone who ran against Gov. Scott Walker in the June recall election and advocated that "Bama," presumably President Barack Obama, be shot to death.