It came in the form of a council by-election in Gipsy Hill, where the Central Hill estate faces the bulldozer against resident's wishes, and where the Upper and West Norwood libraries are also located.
Local people delivered their verdict in no uncertain terms. Labour's vote plummeted 24%, while the Green Party's vote went up 31%.
It is rare to see such a huge swing in London, particularly against Labour's machine politics. I felt privileged to run the Green Party's campaign, with such an inspiring team of volunteers and activists and an outstanding candidate who had recently occupied the Carnegie library in Herne Hill.
It was tantalising to fall short of taking the council seat by just 36 votes. But there is a bigger story to tell.
The more local people look at Labour-led Lambeth council and what it stands for, the less they like what they see. And the more they are embracing a Green alternative.
Yesterday Labour was left in no doubt about what residents think about council policies which know the price of everything, but the true value of nothing. The anger has been steadily building. It was seen at the 2015 General Election during the leadership debates, when a local Brixton resident challenged Ed Milliband over Labour's plans to bulldoze Cressingham Gardens in Tulse Hill. The Labour leader had no convincing answer. The new London Mayor, who came down to campaign for Lambeth Labour during the Gipsy Hill by-election, doesn't seem to have one either.
Local people feel they have become little more than numbers on a spreadsheet. Developers are prioritised over residents. Gyms replace libraries. This is the approach that prioritises pounds and pence over the lives of ordinary people.
The huge swing to the Greens shows a readiness to embrace a distinctive, richer vision which puts people, and the quality of their lives, first. Bulldozing estates and replacing them with unaffordable housing that prices out local people is inefficient and wasteful. It destroys vibrant local communities that have flourished for generations. Closing libraries, to then re-open them as gyms with a few books, misunderstands the vital services, support and spaces that libraries and their staff provide for the local community. Local people know this, and they see in the Greens an approach which takes into account social and environmental, not just economic, value.
They also see a party that is on their side, and wants to share power. On the estates that Labour plans to demolish and replace with less affordable housing, residents, supported by groups such as Architects for Social Housing, have come forward with alternative plans to refurbish their estates and create more truly affordable housing. The story is the same over libraries. The head of the library service put forward a costed proposal that would have kept all ten of Lambeth libraries open.
These plans have won the confidence of local residents. But they have been rejected by a Labour Council, which once promised only to do the things that local residents supported. It is the Green Party that stands for grassroots democracy.
Of course, the distinctive Green Party message has to be delivered strongly and clearly. This is the reality of electoral politics. But this is something that Greens are increasingly in a position to do after the 'Green Surge' before last year's General Election.
Thanks to a burgeoning local party in Lambeth and a dedicated team of Green Party members and volunteers, we were able to implement a targeted strategy in Gipsy Hill. We had thousands of conversations on the doorstep with local residents. We delivered direct mailings to the whole ward and over 40,000 leaflets in a six week period. And when we did, we found our messages were the ones that people had been waiting for.
We may have lost the by-election by 34 votes. A much bigger battle for a distinctive, alternative Green vision, is being won.