Are we witnessing The Great Green Sell-out?

They were idealistic once, but now Caroline Lucas's party is running into the familiar problems of politicians in power. Matt Chorley reports ahead of their spring conference

There was a time when the Green Party would have been manning the barricades, marching on the town hall. But this week the party will take a deep breath and sign off a council budget for the first time, in the face of much gnashing of teeth from their Tory and Labour opponents at the scale of cuts and tax rises in Brighton and Hove. Coming on the eve of the party's spring conference in Liverpool, the budget vote will be a major milestone in the party's coming of age.

Having picked up plenty of middle-class, left-leaning, eco-friendly support, they are now being tested to take tough decisions. Caroline Lucas, the party's first MP, regularly takes to the airwaves to defend the 23 Green councillors who now run the bohemian south coast city. The Conservatives, having once dominated with some 26 seats, are now the main opposition with 18, while Labour languishes on 13 seats.

Labour in particular is deeply bruised. Where once it ran the council and had three MPs, it now has barely a dozen councillors and not one MP. For weeks, the Green cabinet member for finance, Jason Kitcat, was locked in a "Twitter war" with Lord Bassam, a Labour peer who is scathing in his attacks: "They are a fairly visionless group of individuals who are naive and barely competent politicians."

Elected on a manifesto promising "a fresh start for a fair city", the Greens pledged to bring in 20mph speed limits in residential roads, green energy projects in the city, neighbourhood councils, more affordable homes, and to explore the idea of introducing a living wage for council workers. A pilot is being launched to collect household food waste – which makes up a third of rubbish sent to landfill – and developing tourism in the city.

Party officials privately admit to being "shocked" when they were returned as the largest party. But there was a sting in the tail. Central government funding is being cut by about one-third over four years. When the budget was first published in early December, all hell broke loose. Political opponents seized on the admission of the need for cuts, after years of being lectured by the Greens about the need to protect and even increase spending.

Plans to shut some public toilets caused a storm; while every library has been kept open, the mobile library will not be replaced. The idea of Meat-Free Mondays in council canteens caused a major rumpus, with binmen refusing to give up chops, bacon or sausage even for one day a week, forcing a U-turn.

Simon Kirby, Tory MP for Brighton Kemptown and Peacehaven, is one of the Greens' most vocal critics and likens living in the city to "being trapped in a Green laboratory, where ever more madcap experiments are being carried out on a daily basis". Like many of the council's opponents, he also points to the rise in Travellers' camps being set up in the city.

The council was quick to risk the ire of Eric Pickles, the Communities Secretary, by ignoring his plea – and offer of £3m – to agree to a council tax freeze and instead announced a rise of 3.5 per cent. Mr Pickles claims the Greens are running "a cynical and politically motivated campaign using hard-pressed residents as political pawns".

But the Greens insist that to freeze council tax now would store up problems in future. Bill Randall, the council leader, says they approach this week's budget-setting meeting "in pretty good shape". "The big danger about all of this is the future. Where will local government be in four years? It is really worrying." There were redundancies last year, and more are to follow.

Squatters have become a major problem, demanding they be able to take over empty public buildings until they are demolished. Housing remains a significant issue, particularly the amount of accommodation taken up by the city's student population. Alcohol, drug abuse and domestic violence also blight many lives. The council has a disproportionately high number of children in care, including almost 50 under the age of one.

Yet Mr Randall remains upbeat. "There are lots of great things about this city. It is an extraordinary place." Certainly, it has given the Greens their big break. The optimists believe they can build on strong support in Lancaster, Oxford and Norwich to secure more councils. They have high hopes of making another breakthrough in the 2013 elections to the European Parliament, and have their sights on adding a South-west seat to those held in London and the South-east.

But, like the Liberal Democrats in Westminster who saw their popularity evaporate when the rubber hit the road, the Greens could struggle to disprove the old maxim that all politicians are the same.

The green agenda: A dozen things on the to-do list

The Green Party meets in Liverpool next weekend determined to prove that running councils and having their first MP has not dampened their radical zeal. Here are some issues on the agenda:

1 Defend the right to protest.

2 Promote sailing boats over diesel-powered vessels.

3 Ban junk-food adverts before the 9pm watershed.

4 Tax unhealthy food to fund campaigns encouraging people to eat more fruit and vegetables and less meat.

5 Make bankers liable for personal penalties if they wreck whole economies.

6 Support the global Occupy movement.

7 Impose a wealth tax.

8 Remove "loitering or soliciting for the purpose of prostitution" and "brothelkeeping" from definition of sexual offences.

9 Support co-ops and mutuals in a time of economic crisis.

10 Develop major new climate change policy.

11 Tackle the "great train-fare robbery".

12 Keep the NHS public.