GENEVA (AFP) - Swiss voters will on Sunday decide whether to curb immigration by European Union citizens, in a referendum that risks igniting a row with Brussels.
While opponents of the "Stop Mass Immigration" plan have stayed ahead in the opinion polls, their lead has gradually narrowed to a few points.
Neutral Switzerland is not in the EU but is ringed by countries that are members and does most of its trade with the 28-nation bloc.
Since 2007, the EU's 500 million residents have enjoyed an equal footing with locals on the job market of Switzerland, a country of eight million.
That is part of a raft of deals signed with the EU in 1999 after five years of talks, approved by Swiss voters in a 2000 referendum and then phased in.
But a referendum coalition helmed by the right-wing populist Swiss People's Party - the largest in Switzerland's parliament - says that opening the door fully to EU citizens was a huge mistake.
If passed, the coalition's proposal would bind the government to renegotiate the labour market deal within three years.
The government, most political parties and the Swiss business and industry federations warn that ripping it up would kill off the related economic deals and dent Switzerland's credibility as a partner for the EU.
They also say that slapping restrictions on hiring EU citizens would be a disaster, arguing that the steady stream of foreign labour is a driving force of this wealthy economy with virtually full employment and an ageing population.
Until 2007, Swiss firms had to clear bureaucratic hurdles before being allowed to recruit a non-resident, with official quotas for foreign employees set down for each business sector.
Brussels has warned that Switzerland cannot pick and choose from the binding package of deals negotiated painstakingly in the 1990s, seen as a way for Switzerland to enjoy the benefits of access to the EU market without joining the bloc.
Brussels is already battling internal dissent over its own borderless labour market - west European countries complain about competition from citizens of eastern member states - and has ruled out reopening the issue with Switzerland.
Such arguments have not swayed the campaign coalition, which says national sovereignty is at stake.
It argues that the arrival of 80,000 new residents per year has been an economic and social disaster, and not only because EU citizens have allegedly undercut Swiss workers.
It says that overpopulation has driven up rents, stretched the health and education systems, overloaded the road and rail networks, and eaten into the landscape due to housing construction.
Referenda are the core of Switzerland's system of direct democracy, and the coalition mustered more than 135,000 signatures to force a vote.
Immigration and national identity are traditional headline issues in a country with a long history of drawing foreign workers and some of Europe's toughest rules for obtaining citizenship.
But over recent years, the proportion of foreigners has risen from around one-fifth of the population to roughly a quarter.
The majority of recent immigrants are from neighbouring Germany, Italy and France, as well as Portugal.
Most Swiss cast their postal ballots, making voting on the day relatively brief.
Polling stations are scheduled to close at noon (7pm Singapore time) on Sunday, with an early estimate of the results expected soon afterwards.