A stunning election win for Angela Merkel's conservatives in Germany's most populous state is the strongest sign yet that voters have forgiven the chancellor for her open-door migrant policy and are set to hand her four more years in power.

Sunday's triumph in North Rhine-Westphalia marked the third regional election win this year for Merkel's Christian Democrats (CDU), providing them with powerful momentum as they head into a federal election on Sept. 24.

The win reflected directly on the chancellor, who had placed herself at the center of campaigning in a state that has traditionally been a stronghold of her Social Democrat (SPD) rivals.