Political analyst Pascal Sciarini of the University of Geneva cites a "great unknown" in the vote: Will young Swiss who have poured into the streets to lament global warming also turn up at the ballot boxes?
Voters are electing the 200-member National Council, parliament's lower house, and the 46-member Council of States, the upper house, to four-year terms. Recent polls suggest two groups, the Greens and the Liberal Greens, stand to gain seats.
Balloting ends at midday on Sunday. Most voters in Switzerland cast their ballots by mail, avoiding a stop at polling stations. It's an important electoral date for Switzerland, a rich country of 8.2 million where power-sharing and embedded checks and balances make for a stable political landscape, except when it's occasionally shaken up by referendums.
This year has been more dynamic than many: Students and others have marched on public offices repeatedly to echo concerns across Europe about climate change, and the first major women's protest in Switzerland since 1991 drew tens of thousands to demand fairer pay, more equality and an end to sexual harassment and violence. Swiss media report a record number of women are standing for election this year.
Environmental issues resonate here: A group called Glacier Monitoring Switzerland says the Alpine country has lost 15 percent of its glacier volume over the last decade, and warns that all Swiss glaciers could disappear by the year 2100 if warming continues.
Worries about women's rights and the environment are a far cry from the last election, when immigration and relations with the European Union were the main concerns and fanned gains for the populist, right-wing Swiss People's Party that today holds the most seats in parliament.
"There are two main stakes for this election: How significant will the predicted advance of the Greens be," said Sciarini. "And how much will the (Swiss People's Party) lose: That could signal losses for the right generally, and lead to more leftist policies."
The legislature picks the seven members of Switzerland's executive branch: The Federal Council. The Swiss presidency rotates every year among those seven members, who make decisions by consensus — part of the Swiss "magic formula" of democracy that requires different political factions to cooperate, and govern from the middle ground.
Greens don't have any seats in the council now: The People's Party, the Social Democrats, and center-right Liberal party each have two, and the Christian Democrats have one. But big electoral successes could boost the Greens' argument that they would deserve one, too.