POTSDAM, Germany — The Green party candidate seeking to succeed Germany's Angela Merkel as chancellor looked out on a rally seeking more sweeping action to combat climate change. The crowd liked her message. Many are just too young to vote.

And here rests the Greens' short-term problems — but potential long-game strengths.

The Green party is the most popular among German voters 18 to 29 years old, with perhaps even stronger support among those still too young to cast ballots — sometimes dubbed the “Merkel generation” because they have never known a different leader.

Now that Merkel is stepping down after 16 years in power, many Greens supporters fear that a rare opportunity for change was slipping away. Older voters appear to be sticking with Germany’s traditional parties or throwing support behind the far right in Sunday’s election.

“Millions of children and young people live in our country and can’t vote,” the Greens candidate Annalena Baerbock told the crowd in Potsdam’s historic market square on Thursday, urging voters to keep them in mind as they head to the voting booths. “If we keep heading the same way, our children won’t be able to have the same good life.”

Briefly this summer, it looked like Baerbock’s appeals to step up the climate change fight could be a winning strategy. In surveys, voters said climate change and the environment were their top concerns, especially after devastating floods in July in Germany.

But the Greens have since seen their overall approval in decline, leaving many young voters who had hoped for a fundamental shift in German politics exasperated.

Many agree with Baerbock, who has pitched Sunday’s vote as a decision “about the last government that can actively influence the climate crisis” before it’s too late.

The Greens still seem poised to make historic gains. A recent INSA poll showed the party at 15 percent, giving the Greens a shot at becoming part of the next government by joining a coalition.

But, according to the same poll, the party trails the conservatives and the center-left Social Democratic Party by seven and 10 percentage points, respectively.

“The disappointment is big,” said 22-year-old protester Elisabeth Schroeder, joining a rally in Berlin on Friday as part of global events calling for more political attention to climate change.

Not all climate activists in Germany are fully aligned with the Green party, but the vast majority would prefer the party over its two main competitors.

In front of the Reichstag in Berlin, a balloon depicted Armin Laschet — the candidate from Merkel’s Christian Democratic Union — with a slogan mocking his party’s climate policies: “Nothing more than hot air!” Schoolchildren chanted “Laschet, are you still asleep?”

“Many older people are afraid of change. But things will change no matter what,” said Clara Scheich, 18, who is in her final school year.

There is also skepticism of the Social Democratic party’s candidate, Olaf Scholz, who has topped the polls in recent weeks. He has said he would broadly support decisive action on climate change, but critics note the SPD has been part of the government for 12 out of Merkel’s 16 years in power.

“Every time they said, ‘During the next term, we’ll do something about [climate change],’ ” said protester Marcus Schmidt, 30. “But they didn’t do anything.”

The Green party’s initial surge in the polls ended when Baerbock faced a series of questions, including accusations that she embellished her résumé and plagiarized parts of a book.

Baerbock herself said in a recent interview that she would grade her performance at the start of the campaign with a “3 minus” — not a total failure, but not a great success, either.

Delivering one of her final campaign speeches in her home constituency of Potsdam, she appeared to be back in her element. She compared the efforts needed to tackle climate change to the challenges of German reunification about three decades ago.

She stood on a vast square in Potsdam’s historic center that was destroyed in World War II and only rebuilt after the fall of the Berlin Wall in 1989. On the other side of the square, pedestrians sipped beer as they listened to her warnings about a world suffering from drought and floods.

Many climate researchers note that Baerbock is not overdramatizing the time politicians have left to act. “The time window is dramatic,” said Klaus Jacob, the research director of the academic Environmental Policy Research Center in Berlin.

Germany has, at times, been perceived as a leader on environmental action, but it relies more heavily on coal energy than many of its European neighbors. The 2011 Fukushima nuclear disaster in Japan, which prompted Germany to phase out all nuclear power plants, has further complicated the government’s ambitions to switch to more climate-friendly energy sources.

The Green party has faced criticism from climate activists for being too willing to compromise with the parties they blame for those policies. But the Green party leadership has defended that approach as the only possible option in a political system that prizes coalition-building and consensus and has rarely seen a political party gain full control of the Bundestag, the German parliament.

Even if the Greens only come in as the third-biggest party on Sunday, they could shape government policy disproportionately, said Jacob. History provides numerous examples of smaller German coalition partners being able to leave an outsize mark.

“Something similar is certainly possible now,” said Jacob.

In front of the chancellor’s office in Berlin, activists have for weeks been living in tents, seeking a conversation with the three top candidates. Some have been on hunger strikes.

Lea Bonasera, 24, said she joined the group on Monday out of frustration with the climate policies of established parties, including the Green party.

“The climate crisis isn’t being discussed honestly,” she said.

On the main stage nearby on Friday, 18-year-old Swedish climate activist Greta Thunberg echoed a similar message. “No one party is even close to even proposing a pathway that would be consistent with the Paris agreement,” she said.

“Yes, voting is essential — but alone it is not enough,” said Thunberg, calling for more protests.