History says Polanski is going to win the St Pancras by-election

According to the bookmakers Labour is odds-on to win the Holborn and St Pancras by-election. Are they right?

This is a particular kind of contest. When Labour is in government and defends one of its English strongholds, its main opponent has usually been the Conservatives. But from time to time, the challenge comes from its own side of the left-right divide. There have been nine such by-elections in the past sixty years.

The graphic shows what happened. Labour lost five of those seats. Its vote share fell by an average of 24 percentage points.

These huge swings are not hard to explain. When Labour is unpopular nationally, by-elections allow worried progressives to stray without fear. Even if the seat changes hands, the government won’t. A second reason applies specifically to left-versus-left contests, such as Holborn and St Pancras. Progressives can safely change their vote, or stay at home, without even risking handing their seat to Reform or the Conservatives.

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History suggests that Zack Polanski is well-placed to gain Keir Starmer’s old seat from Labour. In 2024, 29% either voted Green or backed Andrew Feinstein, an independent left-wing candidate, while Starmer won 49%. A straight extrapolation from the nine-seat average would give Polanksi a victory over Labour by 52%-25%. He would have to underperform massively not to win at all.

The one historic exception was Southall in 2007. Gordon Brown had just become Prime Minister. Labour’s national support shot up, while the Lib Dems slipped back. This helped Labour hold Southall, albeit with a reduced majority. Andy Burnham will hope for a Southall-style honeymoon win; but even after Labour’s recent rise in support, its poll rating is still around eight points down on the 2024 general election, while the Greens are up around five points. Labour is not as well placed as in 2007.

That said, each by-election has its own dynamic. A Polanski victory is not certain. But if he falls short, any claims he makes to have slashed Labour’s majority should cut no ice. A Labour victory, however narrow, would be a massive achievement for Burnham and his local candidate, bad for the Greens and truly terrible for their leader.