What would be a good night for Labour in the European elections? It is a much harder question to answer than it was for the local elections.

We know what a great night for Labour would be: to finish first, and to make inroads into small towns where it struggled in the local elections. But we don’t know what a good or a bad night is and we won’t until the next election.

Why? Well, because the argument that Labour will make after these elections is that it is a lot easier for voters to express their desire for a pro-Remain position, or a yet harder Brexit position, or a more environmentally-minded political position, or whatever at an election where their vote may actually count for something, but crucially won’t mean passing up the chance to remove the Conservatives from office.

This thesis isn’t stupid and we should at least give it intellectual houseroom. We don’t know if what happened in the local elections was voters going “on holiday” from the main parties, or the beginning of a permanent move. It may well be that come a general election, when control of Downing Street is in play, one or both of the big two parties can very effectively win back any of the voters who went elsewhere in the local elections and any of the voters who will do the same in the European elections.

Equally, it may be that defection is habit-forming: that voters who at the next election have gone on holiday in a local election, a European election, and possibly a mayoral election or a devolved parliamentary election too, decide they actually really like their holiday destination and stick with the Liberal Democrats, the Greens, the Brexit party, Ukip or Change UK at the next election.

We don’t know either way, but it means that it doesn’t really matter how many voters defect from the big parties as we know the answer is “more than zero”. What really matters is the answer to the question: “how many of those defectors can the big parties win back?”, which we can’t answer until the next election.

There are some useful yardsticks, however. The first is that it may give us an indication of where the various new parties and resurgent Liberal Democrats are taking votes. Labour will hope that this shows they are losing votes largely in places where they have votes to spare, while the Conservatives are having the reverse problem. The second is whether, given the inevitable defections both big parties will experience, is whether the Conservatives have significantly more ground to make up than them.