Chancellor Angela Merkel's party secured a decisive win in a crucial state election, exit polls showed Sunday, in a devastating blow to her main rival four months before a national vote.

Her Christian Democratic Union (CDU) clinched a stunning 34.5 percent in North Rhine-Westphalia, a Social Democratic Party (SPD) stronghold for decades. The SPD sank to its worst ever result in the western state with only 30.5 percent, sharply down from the 39.1 percent it obtained in 2012.