That would place them collectively third behind the SVP and the centre-left Socialists (SP), ahead of the centre-right Liberals (FDP), who all have two of the seats on the Federal Council that is Switzerland’s government.
Cabinet seats have been divvied up among the SVP, SP, FDP and Christian People’s Party (CVP) in nearly the same way since 1959.
In December, the two parliamentary chambers will elect the government, but in the past it has taken more than one national election cycle for that selection procedure to change the cabinet lineup to more closely reflect the results.
The Greens’ success could hit all major parties, including the SVP, whose President Albert Roesti has called climate change a “eco-socialist scam against the middle class”.
Analysts caution against expecting too radical a shift after a campaign that was light on typical hot button issues such as migration and Swiss ties with the European Union.
Green parties do not have a monopoly on ecological issues and many voters in the rural SVP heartland are loyal.
GLP founder Martin Baeumle told the Schweiz am Wochenende paper it was an “illusion” to think the Greens and GLP could mount a joint effort to seize a cabinet set. He noted differences in the two parties’ economic and social policy, scant representation in the upper house of parliament, and the difficulty of ejecting a sitting member of government.
Polls close at midday (8pm AEST), with initial projections, including from the many postal votes, due soon thereafter.