Caroline Lucas and co need to retain their following of committed environmentalists while trying to rebrand their organisation as a fully rounded political party

It is a cold, bright day on the Brighton seafront, and, in a modest, low-ceilinged function room at the Metropole Hotel, history is being made.

It is an amiable, unpretentious, occasionally slightly amateurish kind of history, as befits the Green party, and there are only about 35 people there to witness it, but it's history nonetheless: this election, the bookmakers reckon, will be the one at which the party finally gets its first Westminster MP – perhaps even three of them – and the manifesto launch is an unprecedentedly slick affair.

There are multiple camera crews and TV lights and constant, clicking shutters, and a manifesto document with graphic design that rivals Labour's and outshines the Tories'. There are several designer suits in evidence, and very little facial hair. The function room in question is part of something called the Osbourne Suite, which is a bit awkward, but still.

"Put it like this," says a member of the party's media coordination team. (They have a media coordination team!) "Comparing this to the 2005 [manifesto launch] – well, so far, the dog hasn't arrived, and neither has the one man."

The 2010 incarnation of the Green party, under its upbeat leader Caroline Lucas – the MEP who opinion polls suggest will win Brighton Pavilion on 6 May – is engaged in an energetic attempt to have its cake and eat it too. It must retain its following of committed environmentalists while trying to rebrand itself as a fully-featured political party, offering answers to problems that voters perceive as economic, not environmental. And it must persuade voters in its three target constituencies – Lewisham Deptford, Norwich South and Lucas's own – that Green candidates really can win, without making it too obvious that, elsewhere, they almost certainly can't.

"A green vote sends an incredibly powerful message," is how Lucas tries to walk this tightrope. "Even under this hugely undemocratic electoral system, which has been expressly designed to keep out smaller parties ... we now believe we are on the edge of breakthrough."

The resulting manifesto features a suitably contemporary language-mangling slogan – "Fair is worth fighting for" – and an overwhelming focus on the economy, specifically the argument that job creation, rather than "savage cuts", is the route to recovery. Despite far higher taxes on the wealthiest, the party insists that 87% of people would be better off under a Green administration. The weekly state pension would be raised to £170, and the minimum hourly wage from £5.80 to £8.10. (The motorway speed limit would be reduced, incidentally, to 55mph.)

It isn't the Greens' fault, of course, that our electoral system requires the deployment of fantasy concepts such as "a Green administration", as if voters were fervently wondering about how, exactly, that first Lucas-Obama summit might unfold. (They'd probably disagree on nuclear power, for a start.) But, this time round, disillusionment with the main parties means that more voters might at least be willing to entertain the fantasy – while a few, such as the locavore recyclers of central Brighton, have the opportunity to go a step further.

"There is a sense of 'a plague on all their houses'," says Lucas, appearing alongside the Lewisham candidate, Darren Johnson, and 28-year-old Adrian Ramsay, the deputy leader who is running in Norwich South. It has been "really quite moving," she adds, to see voters in the target constituencies realise they might have an alternative. "A party of the left-plus," she calls it, once again suggesting the simultaneous possession and consumption of cakes.

"Look," says Johnson afterwards, over coffee and individual portions of plastic-wrapped shortbread, "it was bloody hard work, years of slog, to break through to the council. A bloody hard slog. But, once we did it, we built on it. And, if we can break through to Westminster with three seats this time, I'd expect dozens of Green MPs at the election after that."

The party still suffers, of course, from the eternal curse of the single-issue party: nobody much cares about your single issue, so people don't vote for you – until eventually everybody cares about it, or claims to, in which case it's hard to make the case that people should vote for you, as opposed to bigger, government-forming parties.

But it surely can't hurt Green chances that the single issue is arguable the single most important issue of all, and that the three main parties are, relatively speaking, so indistinguishable on it and other matters. The immediate political future of Britain isn't going to be bright green: even the party itself uses a washed-out, half-hearted shade of green in its publicity materials, as if it is wary of insisting too much. But the House of Commons is likely to get its first member whose politics match the colour of the benches.