Tegelsbarg, Germany (CNN) German Chancellor Angela Merkel is hoping for another term after more than a decade in power, but she'll have to get past one major obstacle: a sneaker-wearing 28-year-old named Kevin Kuehnert.

Kuehnert is the millennial behind the NoGroKo campaign, short for No Grand Coalition, which has turned Germany's normally staid politics upside down.

Thanks to Kuehnert, Merkel's proposed coalition government between her conservative party and the Social Democrats (SPD) now hangs on a yes or no vote by the SPD's rank and file. More than 400,000 are casting ballots in a postal vote and the results will be announced on Sunday.

"We cannot continue like this: the cozy politics represented by Angela Merkel which does not decide anything," he told CNN."This is now, slowly, ending. This debate in now breaking through in my own party and I believe in society."

Potential blow to Merkel

A "Yes" vote would mean Merkel can breathe a sigh of relief and get back to running the country with the coalition government safely in place. A "No" vote would mean Merkel must scrap the coalition and either take her chances with a minority government or face new elections.

Kuehnert has done all this in his spare time for no pay within three short months of being elected as the SPD youth leader.

"No, I don't have a chauffeur who drives me around," Kuehnert said, comparing himself to other full-time politicians, "but I basically do the same as any other politician in Germany -- only that I do it in my free time."

Germany's September 2017 election was supposed to be an easy victory for Merkel's Christian Democrats (CDU), and she was expected to cruise to her fourth term in office as Chancellor.

Instead, voters revolted against the status quo. Both CDU and the SPD barely maintained their status as Germany's top parties, suffering record losses and losing millions of votes to the far-right nationalist party, Alternative for Germany (AfD), which took nearly 13% of the vote.

Returning to its roots

That's when Kuehnert swung into action. He believes the SPD needs to reestablish its socialist roots as an opposition party rather than be a part of Merkel's government. To join the coalition would cede the leading opposition role to the AfD.

"The differences between the big political parties have systematically become blurred," Kuehnert told CNN. "That will only end up strengthening political parties like the right-wing populists. Not without reason they have integrated the word 'Alternative' into their name because you get the impression that there are no longer alternatives amongst the traditional German parties."

"I think a new grand coalition is playing right into that and this is more dangerous for democracy than possible new elections."

That attitude caught Germany's political elders by surprise. As Merkel tried to hammer out a coalition agreement with the SPD, a senior member of the CDU's sister party, the Christian Socialists (CSU), dismissed Kuehnert's NoGroKo campaign as a "dwarf uprising," a swipe at Kuehnert's youthful inexperience and his height. Kuehnert is 5 feet 6 inches tall.

Kuehnert may be short in stature, but he's long on ambition. In a rousing speech to the SPD's party congress in January he responded: "It's better for us to be dwarves for now so that we may become giants in the future."

Impassioned appeal

CNN met Kuehnert on a snowy day in Tegelsbarg, a sleepy suburb of Hamburg and a traditional stronghold of SPD supporters. The last stop on his NoGroKo campaign was open to the public and held in a community center brightly decorated with children's paintings.

The mostly elderly audience of 30 or so people had coffee and cakes and listened respectfully to Kuehnert's impassioned appeal for a new brand of socialism in Germany. Many had come simply to see Kuehnert for themselves. The crowd laughed as one woman shouted out to demand his age.

SPD youth member Hesam Jozvebayat, who came to see Kuehnert speak, told CNN he is voting to reject a coalition government.

"I was skeptical but I am impressed with you, Kevin," said Peter Wetzel, an SPD member for 52 years. "But I have a question: We have been already without a government for four months. I worry: What happens if we really say no? Will this mean new elections? What does it mean for our country and Europe?''

But others in the audience were willing to risk new elections. Hesam Jozvebayat, a student member of the SPD, voted to reject the coalition government.

''A grand coalition does not allow for debates that are so important to us here in Germany. Instead, it hushes up the move to the right," Jozvebayat told CNN. "If a grand coalition comes together nothing will be discussed out in public anymore. Everything is set in stone and nothing will be challenged. This will only lead to the far-right forces entrenching even more.''

Kuehnert is hoping the party will reject the coalition on Sunday, but said that a "yes" vote would not deter him from forcing change within the party.

SPD youth leader Kevin Kuehnert argues that the party needs to stay in opposition to return to its socialist roots.

"The most important thing for us is a process of rejuvenation," he said. "Over the next few years this party needs to justify why people need us in the 21st century."

Kuehnert's critics may dismiss him as a millennial upstart. But since January, the SPD has signed up 25,000 new members and Kuehnert has plenty of energy, taking inspiration from the likes of US Senator Bernie Sanders and UK opposition leader Jeremy Corbyn.

"They managed to inspire young people with traditional political answers," he said. "Those who were arguing that socialist democracy no longer has a future -- that their ideas belong to yesterday and would no longer excite anyone -- well, they showed us differently. They were able to light a fire for this political idea."

When asked, however, if he has designs on the Chancellery one day, he laughed.

"Oh no! Not to become Chancellor. I wouldn't have any free time to myself. And I like my free time."