Labour's Sagal Abdi-Wali starts favourite in Holborn against Zack Polanski

Holborn and St Pancras votes on Friday, 9 October, in a by-election Labour has treated as a hold and the Greens are treating as a test of their leader. Bookmakers have Sagal Abdi-Wali, leader of Camden Council, as the clear favourite. Zack Polanski, who leads the Greens in England and Wales, is the challenger in a seat Labour has held since it was drawn in 1983.

The seat is Keir Starmer's old constituency, which is why a Green challenge here is being read in Westminster as more than a local contest. A party that takes the former prime minister's seat, or even runs him close, gets a national clip. A party that loses it by a wide margin in a seat with a large Labour machine and a famous name gets a different clip. Polanski chose it knowing the Greens are stronger further east in London, in boroughs such as Hackney and Lewisham, than they are in this part of Camden.

Two events from the weekend sit on top of that map. Green conference voted to define Zionism as racism. Polanski was then barred from entering Israel. In a north London seat with a substantial Jewish population, both facts are already in the canvass. They do not decide a by-election. They give Labour a line it did not have when the writ was moved, and they give Polanski a defensive interview he will still be giving on Thursday night.

The Greens' counter is Gorton and Denton. In February they won that by-election after Labour had said the seat was safe. One upset does not make a method, but it is the reason Holborn is being covered as a contest rather than as a coronation. Two recent polls cited in London reporting suggested that, in a general election held now, the Greens would take no seats in the capital and might finish second in around twenty. Under first past the post, second is nothing. A by-election is the one format in which a concentrated vote can beat that arithmetic.

Abdi-Wali's advantage is institutional. She leads the council that collects the bins and runs the housing list in the borough that contains the seat. A by-election turnout is usually low, and a low turnout rewards the party that can knock on a known door. Camden Labour has that list. The Greens have a national leader on the ballot and a conference vote they now have to explain at every synagogue gate and every tenants' meeting.

The count that will matter on Friday morning is the margin, not only the winner. If Abdi-Wali wins by a few thousand, Polanski can argue that Holborn was the wrong seat and that Hackney or Lewisham is the real target at the next general election. If she wins by the sort of margin a safe Labour seat usually produces, the February upset in Gorton starts to look like a one-off. If he wins, the conference vote and the Israel bar become part of a victory story rather than a liability, and Labour loses a seat it has not lost in forty-three years.

No public poll of the constituency itself has been published this week. The bookmakers' price is a market, not a sample. The conference vote is a text. The election is on Friday.