Tessa Jowell is the current bookmakers’ favourite to be the Labour candidate in next year’s London mayoral election. If the odds are to be believed, Sadiq Khan is the only other person who stands a chance. Diane Abbott, the candidate most likely to benefit from the recent surge in Labour Party membership and support for Jeremy Corbyn, is well behind at 25-1. The Hackney MP shouldn’t be written off just yet – Corbyn was once a 100-1 shot for the party leadership – but the chances of a second bushwhack by the Labour left seem remote.

Which raises an interesting question. Given the relative radicalism of Londoners, who twice elected Ken Livingstone as mayor and furnish Labour with its only safe seats in the south, what happens to the energy behind Corbyn if Jowell, who once said she would throw herself under a bus for Tony Blair, gets the nomination? It’s fair to say that her ‘moderate’ politics won’t foment the enthusiasm that his leadership campaign has generated. With her involvement in the flops of both ‘blue’ and ‘purple’ Labour, she appears committed to any flag the party marches under, so long as it isn’t red.

The limits of Jowell’s potential candidacy can be situated in an international context, too. In recent years, on both sides of the Atlantic, radical political change has taken root at the level of local government. The SNP made its massive gains in the general election in May after it had already won a majority at Holyrood. Syriza’s breakthrough in Greece came in last year’s local elections. In Spain, the radical left found its first electoral expression in June, taking the mayoralties of four of the country’s five largest cities. The scale of Bill de Blasio’s victory in New York two years ago was to some extent the electoral outgrowth of Occupy Wall Street. Earlier this year, the progressive Jesús García mounted a serious challenge to the incumbent mayor of Chicago, Rahm Emanuel. In Seattle, Kshama Sawant is an explicitly socialist city council member who has been one of the country’s most prominent advocates for a $15 minimum wage.

All of this would appear propitious for a candidate of the radical left in London next year. Were either Jowell or Khan to run, it is likely that the favoured Tory candidate, Zac Goldsmith, would prevail. The current favourite to win the Green Party nomination is Sian Berry, their candidate in 2008 when they finished a respectable fourth. In 2012 they beat the Liberal Democrats to third place. What would it take for Berry to have a shot at winning against Goldsmith and Jowell, however unlikely that may be? The answer is that the ‘Corbynistas’ – from Owen Jones, Mark Serwotka and Charlotte Church to the tens of thousands of Londoners who have joined Labour in recent months – would have to walk the walk on political pluralism. Would Jones, for instance, support Jowell over an anti-austerity rival who promised to deal with London’s high-rent, low-pay economy? That’s what the party rules demand, but it would lead to many of Labour’s newer members becoming quickly disenchanted. The British left, to be serious about power, must go beyond tribalism.