Green Party co-leader Jonathan Bartley will tell supporters they can be the most influential force in 21st century politics despite only having one MP.
In his speech at the party's conference in Harrogate he will say that "where we lead, others follow", claiming the Greens had set the agenda on issues including the living wage and opposing austerity.
The party failed to add to its single MP in the general election and saw its vote share fall back from the 2015 result.
But Mr Bartley will say: "Where we lead, others follow. Our London Assembly members forced a Living Wage. Our MEPs stood up for refugees when others stood back. And in Parliament, we have consistently kept climate change on the agenda.
"And you know what, I believe we will be the most influential party in 21st century politics."
Labour under Jeremy Corbyn's leadership has squeezed the space available on the left for the Greens, contributing to the party's poor performance in June.
But Mr Bartley will claim the Greens have shaped the political debate on austerity - and suggest that other parties only "flirt" with changing the economic system.
He will say: "We have changed the minds of other parties on fracking and we are changing the debate about a universal basic income.
"Other parties might flirt with changing the economy. Or the welfare system. But we are the only party that is honest about how much things need to change."
His co-leader Caroline Lucas retained her Brighton Pavilion seat in June and remains the party's sole representative in the Commons.