The crisis that now confronts the Labour Party is difficult to overstate. Had forthcoming boundary changes been in place for the general election, the Conservatives would have won a parliamentary majority of around 50 seats. The SNP has wiped Labour out in Scotland. The rise of both the Green Party and Ukip in England and Wales looks set to continue, and the Liberal Democrats may well take back a few seats in five years time. Labour may well struggle to maintain its current footing in 2020, let alone build on it.

The same problems face Europe’s other historic parties of the centre-left: France’s Parti Socialiste, the Spanish PSOE, Germany’s SPD, Greece’s Pasok. Political crises inseparable from organisational ones reflect a decades-old impasse for social democracy. The constituencies from which these parties traditionally drew both activists and electoral support – the organised labour movement and the working class – have been in flux for years and are unlikely to return to identifiable and electorally solid blocs any time soon.

The Democratic Party in the United States, in contrast to its European counterparts, has thrived over the last decade, mostly as a result of rapidly changing demographics. White Americans are projected to be an ethnic minority before the middle of the century. That dynamic, even with organised labour yet to turn the page on its defeats of the 1980s, has meant an electoral dividend to progressives across the Atlantic. Only one in three white men voted for Barack Obama in 2012. His victory then, as in 2008, was built on significant majorities among women and people of colour. For European social democratic parties to do as well at the polls, a comparable coalition of interests, along with large turnouts, would be required.

All of which makes it particularly strange that immediately after the general election, the Labour Party head office wrote to its members and supporters telling them that the ‘fight begins in Tower Hamlets’. The idea that the fight against a Conservative government wedded to austerity should begin against a left-wing Muslim woman, and a former Labour Party member at that, is instructive of just how deep the party’s problems go.

Rabina Khan, a councillor for Shadwell and cabinet member for housing on Tower Hamlets Council, is running for mayor as an independent against the Labour candidate, John Biggs. There are other names on the ballot, but it will be one of those two who wins on 11 June. The stakes are high. If Labour can’t secure a victory in one of its heartlands a month after a general election defeat, its current problems will look even worse.

When Khan announced her candidacy to a crowd of 2000 people in April – nearly all of them Bengali residents of Tower Hamlets – the crowd responded with jubilation and a resolve to win. That may prove insufficient to beat a national party apparatus focused on a single local election, but Khan and her large base of support is no anomaly.

According to the British Election Study, support for Labour is falling among ethnic minority voters. George Galloway's victory in Bradford West in 2012, and Salma Yaqoob's outstanding results in Birmingham in 2005 and 2010, show that BME voters will desert the party when an alternative candidate runs on the right ticket.

The number of ethnic minority voters is set to grow over the coming decades, and they could prove decisive in national and local elections. Britain will become a majority-minority country at some point this century, if not as soon as the US, and any future success for progressive politics will be intimately tied up with black and brown voters' allegiances. As long as Labour is unable or unwilling to imitate the Democrats, it is unlikely that Yaqoob and Khan's successors will be standing on the party's platform.