The idea has gotten traction ahead of Saturday's EU summit as a way to allow the union's members to integrate at paces they are comfortable with. But it's a contentious issue.
The Netherlands is one of Europe's most liberal countries – and home to one of its most ardently anti-Islam politicians, Geert Wilders, who could capture the most seats in March 15 elections.
With right-wing parties set to do well in elections in the Netherlands, France, and Germany this year, many are concerned about a Europe-wide populist swing. But the EU remains popular.
Prime Minister Renzi resigned after Italians rejected his constitutional reforms. Experts attribute that in part to Donald Trump's election – which could upend elections in France, Germany, and the Netherlands next.
On the campaign trail, President-elect Trump repeatedly challenged the status quo regarding key US allies, including NATO, Japan, and South Korea. But his actual intentions are unclear.