Swiss voters are casting ballots in a referendum led by environmental groups that would require the government to set a precise timetable to end Switzerland's production and use of nuclear energy.

Polls have suggested a close race despite objection. After the Fukushima nuclear disaster in Japan, the Swiss federal government adopted a gradualist approach toward transitioning the country toward renewable energy by 2050. It says nuclear plants should continue to operate as long as they are deemed safe.

The proposal being voted on would shutter by 2029 the last of Switzerland's five nuclear power plants that now generate 40 percent of the country's electricity.

Switzerland regularly holds referendums in its form of direct democracy, which allows voters in the country of about 8.2 million to set policy.