For Mr. Merz and his party, the Christian Democrats, it presents a serious challenge. Since becoming the major force in the coalition government last spring, they have often seemed nervous, panicked and riddled with self-doubt. Their offering to the public has generally lacked both style and substance. But with the Social Democrats — the coalition’s junior party — in electoral decline, it is up to the Christian Democrats to check the far-right charge. The future of German democracy is in their hands.