Against a backdrop of multicoloured pre-fabricated housing blocks, a tanning salon and a travel agent offering last-minute deals to the Baltic coast, Frank Spieth handed out red balloons, pens and advice in equal measure.

The concerns of those who approached his campaign bus in Erfurt, the former communist east of Germany, were primarily local: a woman fighting for compensation from a hospital after contracting MRSA, another seeking a ramp access to her building, a man complaining about the state of windows in the city's schools, which he said needed replacing even before the fall of the wall 20 years ago.

But in little more than a week, when Germans vote for a new parliament, Spieth and his allies are hoping to make a national impact.

His anti-capitalist, pro-social justice Die Linke is striking a chord with an increasingly disenfranchised electorate, espousing causes – such as inequality, reunification issues and, crucially, the war in Afghanistan – that are finding a receptive audience in both east and west.

"Our voters are representative of millions of Germans who feel cut off from the political process and they could have a significant impact on Germany's political landscape," said 62-year-old Spieth, who left the Social Democratic party (SPD) in 2003 after 37 years in protest at its restructuring of the social welfare state.

While Die Linke's rivals have mercilessly attacked it for its radical wealth redistribution plans and its links to the defunct communist regime, its message is clearly getting through.

"The promises [of the mainstream parties] to us about the blossoming landscapes which would follow after unification are mere speech bubbles," said 68-year old Erika Seebach, the MRSA sufferer, in Erfurt. "While some might accuse Die Linke of populism, they get things on to the agenda that really matter."

Polls gives the party about 14%, but after huge gains recently made in key regional elections at the end of August, where it won 21% in the western state of Saarland, Die Linke is being seen as the party that could shake up the political landscape in the 27 September vote.

The policies of bigger parties, including the chancellor Angela Merkel's CDU and its junior Social Democrat partner, are now seen to be disturbingly similar in comparison.

"Generally there are only a few themes that particularly distinguish most of the parties," according to Renate Köcher, joint head of the Allensbach Institute for Demoscopy. "It's only really Die Linke that stands out, in particular for their critical position regarding the German economy and societal order."

Spieth embodies the verve and drive of many in Die Linke. The party, founded just two years ago is, he admits, "a motley crew of democratic socialists, social democrats, communists, Christians, you name it".

Broadly speaking it consists of disillusioned easterners, former members of the ruling communists, and disaffected members of the centre-left SPD.

Die Linke is promising to redress the rich-poor divide by pumping €200bn (£178bn) a year into job creation and financing a gigantic public spending programme, a plan opponents dismiss as unworkable.

Its anti-capitalist stance has raised its profile at a time when expressing such views has become increasingly fashionable, though it has failed to cash in on the economic crisis as growth resumes in Europe's largest economy.

But it came into its own in the aftermath of a recent Nato air strike, ordered by the German military, in northern Afghanistan. The attack triggered a fierce debate about pulling German forces out of Afghanistan. Die Linke is the only party in parliament that is calling for the immediate withdrawal of German troops. As many as 80% of people in Germany are against the Afghan mission.

"It's got people talking about the war, which the other parties had wanted to exclude from the discussion, and that can only be a good thing for us," said Oskar Lafontaine, a former SPD finance minister and one of Die Linke's most prominent leaders.

"The majority of people are against this war due to our own appalling experiences in two world wars but if we don't keep this issue on the agenda, no one will," he told the Guardian at an anti-war rally at the Brandenburg Gate in Berlin.

As he went on stage there were roars of approval as he punched the air and with sweat dripping down his shirt, proclaimed: "We're proud to be anti-war. As Willy Brandt said there should never be war on German soil ever again. That should be the message for now and the future."Christoph Hein, a leading German novelist from Leipzig, who as a pacifist said he was a Die Linke supporter, albeit a reluctant one, put the rise in its popularity down to the increase in floating voters.

"The days when people voted for one party are over. People feel deceived by the other parties, but at least they feel Die Linke speaks their language, and this war issue is a good example of that."If there is one factor holding the party back, it is the claim that it is a home for the "loony left".

It has been accused of not standing up sufficiently to the crimes committed by the East German regime, even for giving a home to people who have called for the reintroduction of the communist-era secret police, the Stasi, or who have compared the Bundeswehr mission with East German soldiers' orders to shoot those who tried to cross the border between the GDR and the West.

The Nobel prizewinning writer Günter Grass, a staunch SPD supporter, has given voice to those who believe that Die Linke has not sufficiently distanced itself from the crimes of the communist era. "Before we could even begin to talk to them they would have to apologise for their persecution of Social Democrats," he said at a recent rally.

A steady rise in the polls and the party's inclusion in regional governments are giving it a stamp of acceptability, but its prospects of national government are slim because its putative allies – the SPD and the Greens – say they are unwilling to accept its demands.

"The problem for the SPD is by opening the gates to let Die Linke in, the SPD would help contribute to their social acceptability," said Manfred Güllner of the Forsa polling institute.

But Petra Pau, one of the first two representatives in the Bundestag of the party when it was in its infancy, insisted Die Linke was not interested in entering government, where its policies would become diluted and wanted to stay in opposition instead where it would have a better chance of enacting change.

"If you look at issues for which I was laughed at when I first put them on the agenda, such as the minimum wage, now they're part of mainstream debate," the politician, who is famous for her spiky red hairstyle, told The Guardian.

"Our aim is to strengthen our position first rather than risk betraying our supporters by compromising on issues," she said.

At the anti-war rally, Ursula Albert, 72, a retired book keeper who spent most of her life in communist east Germany, stood among rainbow peace flags and banners reading "Make Capitalism History" and "Get out of Afghanistan".

Die Linke, she said, was the only party that was trying to redress the inequalities in a reunited Germany, such as the fact that her pension is about 15-20% less than that of a pensioner in the west even though costs were on the same level. "A small group is getting ever richer, child poverty is on the rise. When Germany was divided it was better," she said.