There are probably three basic tests for any political party. The first, of course, is winning power. At the 2010 general election, Caroline Lucas won a narrow 1,200 vote victory over Labour in Brighton Pavilion, pulling together a coalition of the left, students, disaffected Labour voters, tactical Lib Dems, environmentalists and people from the city’s more radical subculture. A year later, 23 Green councillors were elected on a citywide vote share just 1% larger than Labour's, enabling them to form the first Green council administration, albeit a minority one. An efficient election machine, an appealing "anti-politics, anti-cuts" message and ruthless targeting enabled the party to score successive victories against the backdrop of Labour unpopularity, but only on the narrowest of majorities.

Some within Labour, admiring Lucas and the Greens' freedom to express more radical policies, have urged co-operation not competition. Yet the Green message has always been clear. Eighteen of the 23 seats they won were taken from Labour, and they have continued to take aim at Labour even after the Conservatives, who have 18 councillors and two MPs in the city, took office in Westminster. After the Greens' first 100 days in office, their leader said "if we get this right, it will make things very difficult for Labour in the city in 2015." Not better for the city, environment, businesses or residents. Not harder for the Tories. More difficult for Labour.

The second test of a political party is holding office, and what you do with power once elected. What worked in free-thinking opposition soon became unmanageable in government. Having no internal discipline or whip soon saw their election-winning leader replaced, and their new leader commanding a majority of just one over "re-open nominations" in successive internal elections. With no sanction, their councillors have been free to call publicly for their leader to quit. Early on, the "Green Left" started to peel away. A little over a year after standing as a Green council candidate, one member was running against them in a by-election under the TUSC banner.

One Green councillor joined street protests to save a city centre tree, just weeks after she herself voted to fell it to make way for a cycle lane. That same councillor, part of the rebel "watermelon" faction in the Green group, then sought the assistance of the Labour group in trying to oust the Green group leader from his post heading the council. This lead to a local newspaper billboard reading "council calls in counsellors to counsel councillors" as mediators tried to bring the warring factions, now unable to speak to each other, together.

An early and substantial increase in parking fees, one of several heavy-handed attempts to force people from their cars, severely dented residents’ goodwill. Policies widely accepted elsewhere have met fierce resistance in the city, most notably the introduction of 20mph limits as the Green administration has pushed through blanket, unenforced restrictions at a rapid pace. Instead of negotiating a difficult change of terms and conditions to the city’s refuse workforce, the Greens again forced the issue through, leading to a damaging confrontation with the unions and a strike which saw bins overflowing in the streets. Even before the strike, recycling rates, a key municipal environmental measure for any authority, let alone a Green one, were falling.

The public have seen the division among their members, heard controversial statements from their councillors, felt ignored in consultations and seen their flagship "Urban Biosphere" and "One Planet Living" projects as increasingly out of touch with the daily reality of stagnant wages and rising bills. A "no eviction" policy on the Bedroom Tax was widely seen as window dressing in comparison to Labour council policies elsewhere, such as room reclassification.

When one of their councillors quit after the attempted leadership coup this summer, Labour spectacularly defeated them in one of their stronghold wards, wiping out a majority of almost a thousand votes. Caroline Lucas saw her majority disappear in just one of the seven wards in her constituency, the eleven point swing to Labour some ten times that needed to take her seat.

Now a poll by ComRes for the BBC has shown that Green support has fallen by a third since its peak, with Labour opening up a lead of 17 per cent over the party that once vowed to replace them in Brighton and Hove. Damningly, one of the top two factors influencing voting intention was "getting the Greens out".

Perhaps the defining moment for me was speaking to one voter on the doorstep in the by-election this summer. He told me he’d been speaking to friends and neighbours at the local pub, where all agreed that "we gave the Greens a try; now we are coming home to a party we know and can trust." In the contest against a Conservative-led government imposing brutal austerity measures, those who flirted with the Greens are now choosing sides in a national battle where, electorally, the party is irrelevant.

The third and final test of a political party is how well it deals with defeat. Come 7 May 2015 it would appear, from the evidence thus far, that the Greens will face their sternest test yet.

Cllr Warren Morgan (@warrenmorgan) is leader of the Labour and Co-operative Group on Brighton and Hove City Council