"You guys are under full blown assault. This is the greatest assault on the working class people and their unions that I've seen in my lifetime," Biden said.

Biden went on to hammer Romney for comments he made earlier this month suggesting the "message of Wisconsin" was not to hire more government workers. Romney was responding to the president's comments where he suggested the private sector was "doing fine," but that the federal government should help states hire more teachers and emergency responders.

"[Obama] wants another stimulus, he wants to hire more government workers. He says we need more fireman, more policeman, more teachers. Did he not get the message of Wisconsin? The American people did. It's time for us to cut back on government and help the American people," Romney said.

Democrats have argued that the comment indicates Romney opposes both public sector unions and additional funding for teachers and emergency responders.

On Tuesday, Romney told Fox News radio that he thought unions should be considered on a state-by-state basis, but that the Wisconsin vote was in response to excesses that threatened the state's viability.

"You know, I don't think the public is expecting on voting down public unions for police, for fire, for teachers. I think different states have different attitudes and I think that decision will be made state by state. I think in [Wisconsin Gov.] Scott Walker's case that excesses … can lead to a setting where the enterprise in total was in jeopardy," Romney said.

Biden said that Republicans were too quick to vilify public unions, however.

"They really don't understand what we're about. They don't get that you're the most professional, disciplined and well-trained workers the American people can get and deserve," Biden said.