Sahra Wagenknecht, who has never met a political party she could tolerate, has founded her own, and she is shaking up German politics with a combination of right-wing nationalism and left-wing socialism, articulated with seriousness and fluency.

After a career in Communist and left-wing politics, Ms. Wagenknecht, 55, started her own party, the Sahra Wagenknecht Alliance, or BSW by its German initials, only in January. It has taken off like a rocket, running third in three states in elections starting on Sunday, all in the former East Germany, where she grew up.

There, her party is polling between 15 percent and 20 percent, well ahead of any of the three parties in government in Berlin but behind the far-right Alternative for Germany, or AfD, the front-runner in Thuringia, Saxony and Brandenburg. Nationally, her party is polling as high as 9 percent, forcing her way into the conversation before federal elections in September 2025.

Germany’s mainstream parties are losing their dominance. Ms. Wagenknecht has benefited from the crumbling of the old order. In Germany’s new political universe, she is something of a loose electron, difficult to characterize, adding volatility and further precipitating the breakdown of a political spectrum where left and right were once neatly and predictably arrayed.

Though coming from the left, her strength derives in part from sharing many of the same positions as the AfD, including calling for a sharp reduction in migration and the end to aid for Ukraine, but without any neo-Nazi tinge.