Green Party leader Caroline Lucas has said the party can make an "historic breakthrough" at the general election.
She told members at the party's spring conference in London they could win "one or two seats" and "contribute to the renewal of politics".
But she said they should be clear about what they could achieve, adding: "We won't form the government."
Ms Lucas said the Greens were the best party to defend the NHS against privatisation and to tackle inequality.
'Leadership in ideas'
Ms Lucas, who is one of the party's two MEPs, told the conference she wanted to talk about "victory".
"I am going to throw caution to the winds," she said. "I believe that on May 6 we can make a truly historic breakthrough. We will win our first seat in Parliament."
We can survey a sea of dashed hopes and missed opportunities
Caroline Lucas, Green Party
She highlighted her own Brighton Pavilion seat as a potential win, along with Norwich South, Lewisham and Cambridge.
She said the party would "not have a seat in cabinet, or be part of a coalition" or be "given ministerial portfolios", but it could play a key role in holding the government to account and by showing "leadership in ideas".
Ms Lucas did not make environmental issues the number one focus of her speech, instead choosing to label the "privatising of our National Health Service" as Labour's most "shocking" legacy.
She said neither the Tories nor Lib Dems would reverse that trend but the Greens would prevent hospital closures, protect public sector jobs and rule out further private sector involvement.
She also accused Labour of turning Britain into "a country of inequality" and criticised ministers for failing to take action to tackle climate change or introduce "clean technology".
"And so, as we come to the final weeks and days of New Labour in government, we can survey a sea of dashed hopes and missed opportunities...
"And what will come from Cameron? More of the same, but laced with a Blairite smile.
"For his brand of Conservatism means taking the worst of Labour and pulling it down to new depths."
Ms Lucas gave her backing for a so-called Robin Hood tax on bank transactions and praised the work of the Iraq inquiry.
But she called for Labour MP Kim Howells to stand down as chairman of the Commons' Intelligence and Security Committee (ISC) following the row over the case of UK resident Binyam Mohamed.
The former Guantanamo Bay detainee claims he was tortured with the knowledge of British security officers.
"If the chair of the ISC cannot conceive of the possibility that MI5 may have gone off the rails, then he is not fit to head the watchdog tasked with its oversight," Ms Lucas said.
MI5 director general Jonathan Evans has denied allegations the security service did not respect human rights and said his officers operated within the law. He also said allegations it had tried to cover up its activities were "the opposite of the truth".
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