Swiss voters are casting ballots in a referendum Sunday on whether to hasten the phasing out the country’s nuclear power plants. Polls suggest a tight race on an issue being championed by environmentalists that could put Switzerland on a track similar to neighboring Germany. If the “yes” vote prevails, Switzerland will close three of the country’s five reactors next year and the remaining two by 2029. Following the 2011 disaster in Fukushima, Switzerland, like other countries, pledged to decommission its aging reactors, which generate 40 percent of Switzerland’s electricity, as they reach the end of their safe operational lifespan, but it did not specify a date. As part of an energy plan that runs through 2050, the Swiss government has agreed not to replace its existing nuclear plants, which can operate as long as they are deemed safe. Anti-nuclear advocacy groups, such as the Swiss Greens and Social Democrats, have said the Swiss government’s timetable has not been fast enough and pushed for the referendum this weekend that could speed up the planned exit. Because of public concern, no new nuclear power stations have been built in Switzerland since 1984.