BERLIN — The man who could topple Chancellor Angela Merkel wears hooded sweatshirts, loves amateur soccer and has yet to finish his politics degree. His nickname is “baby face.” His real name is Kevin Kühnert.

Mr. Kühnert, who at 28 runs the youth wing of Germany’s center-left Social Democratic Party, has been touring the country to convince fellow party members to vote down a coalition deal that would keep Ms. Merkel in power for another four years — and his own party in place as the junior partner to her Christian Democrats.

Three months ago, few people had heard of Mr. Kühnert. Now, his boyish face with its rebellious cowlick is ubiquitous. He is a regular on prime time talk shows and routinely described as a rising star of German politics.

But he is also an enfant terrible for Europe’s political elites, who breathed a sigh of relief when Ms. Merkel announced last month that she had secured a governing deal, seemingly ending five months of political limbo in Europe’s most important capital.