"Sen. Kerry's statement ... in Illinois was unbecoming of a candidate for the presidency ... and ... we call on Senator Kerry to apologize to the American people for this negative attack," Marc Racicot, chairman of President Bush's re-election campaign said in a statement.
Unaware his microphone was still on, Kerry made the remarks while chatting with workers at a Chicago ductwork and piping manufacturing company following a videoconference call to a meeting of AFL-CIO leaders in Florida.
"We're going to keep pounding, let me tell you. We're just beginning to fight here," Kerry said. "These guys are the most crooked, you know, lying group I've ever seen. It's scary."
But no retraction was forthcoming, the Chicago Tribune said.
Kerry spokesman David Wade offered no apologies for the rhetoric and said Kerry was referring to Republicans' "hardball, gutter politics."
"We're going to make it very clear that he's a Democrat who punches back," Wade said.
2014: The Year in Music [PHOTOS]