It is the Oldham West and Royton by-election today. It is one of Labour’s safest seats, with a majority of almost 15,000, and the party has one of the best by-election candidates it has had in years in Jim McMahon, the popular leader of the local council. So should we be worried about the result?

No one seems to think Ukip can win the seat. But a collapse of the Labour vote would be deeply worrying.

The constituency is made up of three main groups: first, South Asian communities, which make up around 20% of the electorate. Second, middle-income households, accounting again for around 20% of the electorate. And finally, white working class communities. (If you’re psephologically inclined there are a couple of good analyses of the constituency on the Guardian and Progress websites.)

Middle-income households will largely vote Tory or UKIP. The Asian communities will stick with Labour – they certainly will not go UKIP. So the danger for the party is that the white working class deserts it. That would spell disaster across the north, particularly in seats where UKIP is in second place behind Labour.

5,000 plus majority: no need to worry

So what should we look out for? Peter Kellner has the best guidance here. He expects a much lower turnout than the general election and thus a much smaller Labour majority. A repeat of the pattern of by-elections in safe northern seats during the last parliament would see Labour’s majority cut to around 5,000. No need to worry.

Majority of less than 4,000: worrying

Kellner regards a majority of less than 4,000 as a terrible result. Labour would have lost more votes than in any English by-election during Ed Miliband’s time as party leader. He thinks the party should be very worried indeed about its chances in the May elections.

Majority of less than 2,000: extremely worrying

Kellner estimates that McMahon’s local popularity is worth between 1,000 and 2,000 votes. The town has been linked to Manchester by the Metrolink tramline and seen extensive regeneration under his energetic municipal leadership. McMahon is also likeable and charismatic. So a majority of less than 2,000 means Labour would have lost the seat without the Jim-factor. Extremely worrying.

Kellner does not dwell on the possibility that Labour might lose the seat. That would clearly be a terrifying prospect for the party. Lets hope the Labour vote holds up.