A bugging device has been found in the office of Philadelphia's Democratic mayor, Mr John Street. Democrat.
The bug, described by police as having multiple microphones, prompted Pennsylvania Democratic Governor Edward Rendell and Republican Senator Arlen Specter to call on federal authorities to say whether the device is part of an FBI investigation.
The FBI refused to confirm or deny an inquiry.
Mr Street, who is involved in a bitter election campaign against Republican Mr Sam Katz, has denied any wrongdoing at a news conference.
Mr Katz described the suggestion of political foul play by his Republican backers as "reckless" and "unfair."
The Republican-appointed US attorney-general was also forced to defend himself against suggestions that his office ordered the bugging in a Republican Party effort to help Mr Katz.
US Attorney Patrick Meehan, a Bush-appointed official, insisted his office stays out of partisan politics. "That was the practice of my predecessor and that is my practice as well," he said.
But Mr Street's campaign spokesman said: "Do we believe that this is out of the realm of possibility to have been perpetrated by the national Republican Party to help Katz and Bush? We believe it is absolutely within the realm of possibility."
Four years ago, he lost the mayoral race by several thousand votes, and this year's rematch has been marked by contoversy, including accusations that supporters of Mr Katz, who is white, have used race as an issue against Mr Street, who is black.
The mayoral election is scheduled for November 4th. Political analysts have suggested a Republican victory in the heavily Democratic city could provide President Bush with an edge for winning Pennsylvania in his 2004 re-election bid.