The Swiss were voting on Sunday on limiting immigration from the European Union, which, while not expected to pass, has sparked fears a shock "yes" could devastate relations with the bloc.
The initiative, backed by the populist right-wing Swiss People's Party (SVP) -- Switzerland's largest party -- has lost public support in recent polls.
The latest survey showed 65% of those questioned opposed the call to tear up an agreement permitting the free movement of people between Switzerland and the surrounding European Union.
It seems unlikely the initiative will garner the double majority needed to pass, winning both the popular vote and most of the 26 cantons.
But the SVP has eked out surprise victories in the past in its war against tightening relations with the EU, fuelling concern that Switzerland's relationship with its biggest trading partner could be in jeopardy.
Most people vote in advance in the popular polls and referenda held in the country every few months as part of the direct democratic system, and ballot boxes open for just a few hours.
Polls open at different times in different cantons, but will all close by 1000 GMT, with initial results expected by early afternoon.
High voter participation?
Voter participation is generally low in Switzerland's popular polls, and rarely inches above 50%, but a far higher voter participation than usual is expected this time, with some cantons anticipating as high as 60%.
SVP's initiative calls for Switzerland to revise its constitution to ensure it can autonomously handle immigration policy.
The party, which has built its brand by condemning immigration and EU influence, warns that the wealthy Alpine country is facing "uncontrolled and excessive immigration."
While not an EU member, Switzerland is bound to the bloc through an array of intricately connected bilateral agreements.
If the SVP initiative passes, authorities would have one year to negotiate an end to the 1999 agreement on the free movement of persons between Switzerland and the bloc.
The proposal goes even further than a similar measure, also backed by the SVP, that narrowly passed in February 2014, demanding that Bern impose quotas on the number of work permits issued to EU citizens.
That vote threw Swiss-EU relations into disarray, with Brussels warning any curbs on immigration by EU citizens would put a whole range of bilateral agreements at risk.
Bern struggled to find a way to respect the vote without permanently alienating EU neighbours, in the end reaching an agreement in 2016 that stopped far short of an initial quotas plan, which Brussels had fiercely rejected.
Instead, Bern opted merely to require Swiss employers to jump through a few bureaucratic hoops before hiring from the bloc.
The SVP condemned that compromise as a "betrayal" and launched its new initiative.
Votes on the SVP's initiative and several other issues had been scheduled to take place in May, but were postponed since the coronavirus lockdown measures prevented campaigning.