Britain's only Green MP, Caroline Lucas, is to step down from the leadership of her party as part of a strategy centred on challenging the Liberal Democrats at the next election.
Ms Lucas, the MP for Brighton Pavilion, wants the party leadership to be used as a vehicle to boost the profile of other potential Green MPs. The Greens think they can win more Westminster seats in 2015, especially through the disenchantment of Lib Dem voters unhappy with measures Nick Clegg and his party have supported in the Tory-led Coalition, such as tuition fees.
They sense a historic opportunity, should the Lib Dem vote collapse, to take over as the alternative to Labour and the Conservatives.
"There is a huge opportunity," Ms Lucas said. "People who voted Lib Dem in the past feel betrayed ... A lot of them are looking for a new home and we want to be able to welcome them."
The first-ever Green MP when elected in 2010, Ms Lucas intends to stand again at the next election, but at the weekend she notified the Green Party executive that she would not stand for election as leader when her second two-year term expires in September.
She said: "I want to ensure that we use the leadership of the Green Party in a strategic way, to help us build momentum and build up our electoral presence."
The Greens have 150 UK councillors and in the four years since Ms Lucas became leader the party membership has doubled, to 16,000. There are two Green MEPs, Jean Lambert in London and Keith Taylor in the South-east, and the party has a realistic chance of more MEPs in the 2014 European elections.
But the real prize remains greater representation at Westminster. In offering a radical alternative – as well as pushing environmental policies on issues such as climate change, they oppose nuclear weapons and the time and scale of spending cuts – the Greens think they are well-placed to supplant the Lib Dems and have already begun their campaign to do so.
Their spring conference was held in Mr Clegg's seat of Sheffield Hallam and the autumn conference will be held in Bristol and targeted at the West Country as a Liberal Democrat stronghold.
Ms Lucas said that the Greens were now "absolutely" going after the Liberal Democrat vote.
"We don't want to be complacent about it, to assume we have any right to that (Lib Dem) vote," Ms Lucas said. "But we do want to be able to take our message to Lib Dem areas in particular, to make the case that so many of these issues which we know they care about, the kind of manifesto that they thought they were voting for, are key principles of the Green Party."
Who's next? Leading candidates
l Adrian Ramsay The Deputy Leader got 15 per cent of votes in Norwich South at the election in a seat the Lib Dems held.
l Jean Lambert Lambert has been a Green MEP for London since 1999, successfully broadening the policy base of the party.
l Keith Taylor He has been MEP for the South-East region since 2009, taking over from Caroline Lucas.
l Jenny Jones Green candidate in the London mayoral elections who beat Lib Dem Brian Paddick into fourth.