Australia's opposition Labor leader has been forced to defend his stance on what sort of climate change treaty should follow the Kyoto Protocol.
Rudd has been leading in the polls so far
Kevin Rudd says he will not ratify an agreement that does not include China and India, apparently contradicting his party's environment spokesman.
Mr Howard has seized on the apparently embarrassing reversal, saying it shows Mr Rudd's policies are the same as his.
Mr Rudd has promised to sign Kyoto if he wins November's election.
The government's refusal to do so has been deeply unpopular and a damaging issue for Mr Howard in the election campaign.
But the trouble for Labor began in discussions about what sort of agreement should follow Kyoto when it expires in 2012.
Labor environment spokesman Peter Garrett suggested that he would sign any new agreement regardless of whether it included India or China.
Mr Rudd later denied this, saying any such agreement would be unworkable without the two emerging giants.
Correspondents say this is welcome news for the prime minister, for whom environmental issues have so far been a major vote-losing issue.
"I've got to say that the most unbelievable capitulation of this election campaign so far is that Mr Rudd now agrees with me on climate change," he said.
"I don't agree with Mr Rudd. He has agreed with me."
Mr Garrett, a former rock star, later retracted his remarks although his political future if Labor wins the election is now in some doubt.
Mr Rudd has been consistently ahead of the prime minister in the polls.
Mr Howard, seeking a fifth term in office, will be hoping this confusion will at least neutralise the environment issue.
This would enable him to draw voters' attention back to his record in office, which has been the focus of his campaign.