John Prescott has mocked Charles Kennedy for being tired following the birth of his son Donald.
The deputy prime minister, out on the campaign trail, congratulated the Lib Dem leader on becoming a father.
But he added: "Why is he exhausted after one day of looking after the baby - what does he think mums do in Britain, knock off after 24 hours?"
A Lib Dem spokesman branded Mr Prescott's remarks "cheap and nasty" and said it showed he was worried.
Election stump jibe
"The Tories can't win and he knows we are increasingly a threat," the spokesman added.
Mr Prescott went on to refer to the news conference where Mr Kennedy appeared to forget his own tax plans.
"So he's exhausted, he's tired. Did you see him on the telly? 'I can't answer that question on the council tax because, what was it? Am I doing that?'," Mr Prescott joked.
He then added: "He didn't know because because he hadn't thought it through."
Mr Kennedy appeared to fluff his lines when asked a detailed question about his party's tax plans at the launch of the Lib Dem manifesto within hours of his son's birth.