Politics in the centre ground should not be seen as the only option to “toxic” alternatives, according to Patrick Harvie.
The Scottish Green Party co-convener will speak at his party’s conference in Edinburgh later on Saturday, when he is expected to say they can offer a positive vision to voters.
Mr Harvie will also tell supporters the party must look to broaden its appeal as it looks to make gains across Scotland.
The UK is not only able, but fully willing, to retrospectively cut Holyrood's powers and block the laws we make whenever they disagreePatrick Harvie, Scottish Greens co-convener
“We need to be ready to win more Holyrood seats, building on the gains we made in 2016 and filling in those gaps on the map where three regions of Scotland don’t yet have a Green MSP,” Mr Harvie is expected to say.
“We need to broaden the appeal of Green ideas to every part of our country, because every single one of us is involved in the greatest challenges of our age, and every single one of us is being failed by middle ground politics – or worse, by the idea that something moderate is the only alternative to something toxic.
“A myth has been allowed to develop that only the far-right populists are gaining ground in Europe.
“But while they do pose a serious threat, the truth is that in many European countries it’s the Greens people are turning to for a positive vision of the future.”
Mr Harvie will also address members on global challenges, taxation measures and on constitutional issues.
He will say: “It has been made clear that now – 20 years after the people of Scotland finally achieved their own parliamentary democracy – whatever laws we pass in devolved areas of authority, the UK is not only able but fully willing to retrospectively cut Holyrood’s powers and block the laws we make whenever they disagree.”
The party’s co-convener, Maggie Chapman is expected to praise the campaigning efforts of young people who walked out of school to demand action on climate change.
She is also set to claim the Greens are the only party willing to radically shake up democratic structures.
“We have a huge opportunity to support the school strikes as a social movement,” Ms Chapman will say.
“Our young people are leading the way in challenging the systems and structures of our politics and our economy. We must listen to them.
“The age of individual action is dead. Now is the time for social movements to seize power.”
She will add: “Other parties think that change happens only by winning power and then imposing change on the people.
“But Greens know, because of our roots in participatory democracy and our experiences in communities, that real change is different.
“It is about supporting and being a part of building wider social movements, bringing people in, and working constructively to create change.”
The party will also hold a hustings on Sunday as part of the candidate selection process for the EU election.