BERLIN • Germany's Greens are open to working with any party except the right-wing Alternative for Germany after the September election, but will list gay marriage as a condition, which could make it tricky to work with Chancellor Angela Merkel's conservatives.

The Greens, which also named climate protection as the party's price for participating in government, could be a kingmaker in three potential coalitions after the Sept 24 election. Still, recent polls have shown support for the party slipping to between 6.5 and 8 per cent, after having spent much of last year above 10 per cent.

During its party congress at a velodrome in Berlin, the roughly 800 delegates declared they would not sign a coalition deal unless the terms allowed gay marriage - a step up from the civil partnerships allowed since 2001.

"Unless the discrimination against lesbians and gays on this point ends, you can't count on our cooperation," said Greens politician Volker Beck, who proposed the motion.

Delegates also decided to demand that the 20 dirtiest coal-fired power plants be closed immediately and that no new vehicles with combustion engines be allowed from 2030.

In a speech, Ms Katrin Goering-Eckardt, one of the party's two top candidates, lashed out at United States President Donald Trump for abandoning the Paris climate accord, saying he had "climbed into the ring against the earth" and the Greens would "take on this fight".

Drawing applause, she sent Mr Trump a tweet featuring a photograph of delegates holding up posters that spelt out "climate first" while others held cartoon cut-outs of a red-faced Mr Trump and pictures of a frowning, sweating planet.

Mr Cem Ozdemir, the Greens' other top candidate, said he and Ms Goering-Eckardt would not sign a coalition agreement that did not lay out the rules of climate protection, adding that this meant phasing out coal.

Politics professor Oskar Niedermayer at Berlin's Free University said the party had no unique selling point when it came to the environment - at a time when Germans are more concerned about immigration, security and terrorism. "The Greens don't draw big crowds on those issues," he said.

Mr Alfons Hener said he was concerned about the party's slump in polls, and that the Greens had become "too harmless" and "too old" as too few young people had joined. The retired civil servant, one of the party's founding members, said he finds it hard to explain how the Greens - once characterised as "anarchists, Communists and organic eaters" - are different from the other parties.

"I can't find any words on that any more, and that annoys me about the Greens," he said.

REUTERS