As the six-week campaign reached the halfway mark, Howard's conservative coalition and the opposition Labor Party engaged in a frantic bidding war of environmental pledges.

Opinion polls put the Green vote at about six percent, well above the 2.2 percent swing required to unseat Howard.

Howard this week pledged two billion dollars (1.4 billion US) to save the country's ailing river systems, prompting Labor leader Mark Latham to respond with a billion-dollar river package of his own.

The prime minister also said his yet-to-be-released forests policy would stop logging in Tasmania's iconic old-growth forsts, only to have Latham pop up on a Perth beach launching a multi-million dollar policy to keep the nation's coast clean.

At the same time, Howard's government maintained its attack on the Greens "kooky" non-environmental policies in a bid to stop disaffected coalition voters switching to the minor party.

Deputy Prime Minister John Anderson said voters should realise the Greens were "like a watermelon, green on the outside and red on the inside".

Environmental groups said the sudden conversion to their cause was due to internal party polling that showed green issues were increasingly important to the swinging voters that will decide the election.

"We're very cynical about the approach of the big parties on the environment but if we can use the situation to force them into some meaningful commitments, we'll do so," Greenpeace Australia campaign director Danny Kennedy told AFP.

Greens senator Bob Brown said Howard's environmental credentials were damaged by his refusal to ratify the Kyoto protocol on greenhouse gases and his past support for logging in the island state of Tasmania.

The Tasmanian forests -- vast wilderness areas containing some of the oldest trees on the planet -- are looming as the major green issue of the campaign.

Already 104 British politicians and 101 Australian scientists have signed open letters to Howard calling for an end to the logging of old growth forests for woodchips.

Howard said earlier this month that he wanted to stop old growth logging, but not at the cost of timber workers' jobs.

Speculation on how he will attempt to achieve the goal has centred the possibility of compensation payments to the industry in return for an end to logging in areas of high conservation value.

Estimates of how much such a package would cost vary wildly, the Greens say about 90 million dollars, the timber industry nine billion dollars.

Labor has also vowed to save the Tasmanian forests and the state's pro-logging Premier Paul Lennon said his constituents were sick of their environment being used as a political pawn by federal politicians hoping to win over tree huggers in mainland electorates.

"This issue has nothing to do with science, it has nothing to do with biodiversity, it has nothing to do with forests that are worthy of protection under some scientific criteria," the Labor politician said.

"It is pure and simple an issue about the prime minister wanting to ensure that he gets more than 50 percent of Green preferences in selected marginal seats in mainland Australia."

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