A SIMPLE misunderstanding almost stopped Nafsika Butler-Thalassis’s political career before it started. “I hadn’t actually realised I could stand,” says Ms Butler-Thalassis, a Greek national who is running to be a Labour councillor in Westminster. Brexit provided a swift, if unwelcome, education for Britain’s 3m EU migrants about what rights they enjoy. Local elections taking place in parts of England in May will be their first chance since the Brexit vote to vent their feelings at the ballot box.

A million votes from EU nationals are up for grabs, according to the Liberal Democrats. About 400,000 of them are in London, where nearly all seats are up for election and several councils are expected to change hands. In Westminster, which Labour hopes to win from the Conservatives for the first time in the borough’s history, nearly one in five residents comes from elsewhere in the EU. Their votes could prove crucial.

But winning them over may not be easy. Unlike Commonwealth and Irish citizens, EU nationals cannot vote in general elections. When canvassing, the first task is to convince them that they are eligible to vote in local ones, says Ms Butler-Thalassis: “The first thing they say is, ‘I can’t vote’.” The fact that a Greek is standing for election soon convinces them that they can, she adds.

Wider campaigns encouraging European migrants to vote have begun. The 3 Million, which lobbies for the rights of EU citizens in Britain, is to launch a registration push. The Lib Dems have released videos in 21 languages, with foreign MEPs imploring their countrymen to turn out.

In London, which voted heavily for Remain, campaigns are a mish-mash of macro-concerns about Britain’s future outside the EU and micro-moans about parking and planning. Labour’s Brexit policy of constructive ambiguity is replaced in the capital by full-throated support for the bloc. In Hammersmith, where 19% of the population are EU nationals, the Labour leader of the council poses with an EU flag on the council’s website. Conservative-led councils have strained to distance themselves from the Tory government, declaring that EU migrants are not “bargaining counters”.

A small swing could make a big difference. In local elections, with their low turnout, a few hundred votes can change the make-up of a council, points out Gareth Roberts, who leads the Lib Dems in Richmond. Only 39% of people voted in London’s most recent local ballots, in 2014. In Westminster turnout was 32%. With the rights of EU nationals after Brexit still to be decided, “this could also be their last opportunity” to vote, Mr Roberts adds. The clout of European migrants will ultimately depend on whether they have adopted a proud British tradition: not bothering to vote in local elections.