BRUSSELS — With more than 400 million Europeans across the 28 countries of the European Union entitled to vote in the European Parliament elections that end on Sunday, the poll is, next to India, the largest democratic exercise in the world.

But since these elections began 40 years ago, when the bloc was only 15 countries, turnout to vote for the Parliament — the bloc’s only directly elected branch — has decreased every five years.

Turnout was among the key indicators: Would it decline below the 42.6 percent of 2014, or would Europeans respond to calls from both populists and mainstream politicians who suggested that this election was important for the future of Europe?

[Follow our coverage of European election results.]

Over all, preliminary figures later on Sunday showed that for 27 nations, excluding Britain, turnout was nearing 51 percent, according to the European Parliament spokesman, Jaume Duch Guillot, the highest in at least 20 years. Turnout was also higher this year than five years ago in key countries like France, Germany, Poland and Spain. It was roughly the same in Italy and marginally higher in Slovakia, where traditionally less than 20 percent of eligible voters bother to vote. In France, turnout was projected to break 50 percent for the first time since 1994.