Nick Clegg today urges voters who care about the environment not to "waste" their votes on the Green Party.
The Liberal Democrat leader, writing in The Independent on Sunday, says only his party can make a "radical" difference on climate change.
Mr Clegg made the appeal as his party's annual conference got under way in Bournemouth with the Lib Dems desperate to make an impact before the election.
And in an interview with the IoS, Vince Cable, the deputy leader and Treasury spokesman, enters the debate about spending by warning that "every bit of the public sector" must "justify its existence". Mr Clegg said yesterday there must be "savage" cuts to public spending.
Mr Cable also holds out the prospect of, in the event of the Lib Dems holding the balance of power in a hung parliament, the party propping up Labour to "maintain stability".
"We are willing to sit down and talk to Labour and the Tories without favour, and decide how best to deal with the situation in the national interest."
The Greens have two MEPs but no MPs. In 2005 they polled just 1.07 per cent of the popular vote and contested 200 seats.
Mr Clegg wrote: "I understand completely why people are drawn to the Green Party ... they know the Greens can't win, but they want to send a message that the environment matters.
"I understand that ... but only a party with real power ... can make a radical difference."