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

Top candidate Katrin Goering-Eckart of Germany's environmental party Die Gruenen (The Greens) holds a speech at a party congress in Berlin, Germany June 17, 2017. REUTERS/Hannibal Hanschke

The Greens, which also named climate protection as their price for participating in government, could be a kingmaker in three potential coalitions after the Sept. 24 election, although recent polls have shown their support slipping to between 6.5 and 8 percent after spending much of last year above 10 percent.

At their party congress in a velodrome in Berlin, the roughly 800 Greens delegates declared they would not sign a coalition deal unless it allowed gay marriage, a step up from the civil partnerships Germany has 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 close immediately and that no new vehicles with combustion engines be allowed from 2030.

In a speech Katrin Goering-Eckardt, one of the party’s two top candidates, lashed out at U.S. 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”.

As she drew applause, she sent Trump a Tweet featuring a photograph of delegates holding up posters spelling out “climate first” while others held cartoon cut-outs of a red-faced Trump and pictures of a frowning, sweating planet.

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

A recent poll by Infratest dimap showed 57 percent of Germans think the Greens are not very important as the party’s political rivals now address environmental and climate issues.

Oskar Niedermayer, politics professor at Berlin’s Free University, said the Greens had no unique selling point when it came to the environment - at a time when environmental issues are also “totally unimportant” for Germans who are more concerned with immigration, security and terrorism.

“The Greens don’t draw big crowds on those issues,” he said.

Alfons Hener, a retired civil servant, 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.

One of the party’s founding members, he said he finds it hard to explain how the Greens, who he said were once characterized as “anarchists, Communists and organic eaters”, are different to the other parties.

“I can’t find any words on that anymore and that annoys me about the Greens,” he said.

Greens member Benita von Brackel-Schmidt said the party needed to make clear to voters how its topics affect their lives.

Ozdemir told Reuters the polls were not decisive, the election result would decided on Sept. 24 and the Greens were stepping on the gas now.

“It’s necessary to be fit for the next 100 days. I often go cycling and occasionally do yoga too so my fitness is good enough,” the 51-year-old politician of Turkish origin said in an interview.