‘Emily Thornberry’s tweet may be the most devastating message Labour has managed to deliver in the past four years.’ Photograph: David Levene

It may be the most devastating message Labour has managed to deliver in the past four years. It’s already being described as the party’s “47%” moment – a reference to the observation that nailed shut the lid on Mitt Romney’s presidential campaign, when he dismissed the 47% of American voters who wouldn’t ever back the Republicans.

It is really quite hard to come up with a more lethal tweet to send out to the party’s core vote on polling day.

Up to now the best thing about the Rochester byelection has been that it has been a contest between Ukip and the Tories, and the Tories have had their backs against the wall. For all that it is a seat where Labour should be making a show – until the last election, many of the people likely to vote Ukip used to elect Bob Marshall-Andrews, the MP from 1997 – at least Labour could comfort itself that whatever happened would be worse for the Tories than for them.

That was until the MP for Islington South got her phone out. Emily Thornberry, the shadow attorney general, should have stuck to getting the vote out.

Instead, with one tweeted image she has turned the fire back on to Labour.

So much for Labour’s fight for hearts and minds. So much for this week’s efforts by shadow cabinet members like Yvette Cooper and Rachel Reeves, launching policy initiatives tailored to persuade these voters, and millions of others like them, that Labour feels their pain.

One click, just one click, that’s all it takes. Ed Miliband’s Labour is once again the party of the metropolitan elite.