Since March 7, 2011 he is chairman of the CDU parliamentary Group in the Hamburg state legislature (Hamburgische Bürgerschaft), the largest opposition party.

On April 1, 2004, Wersich became a member of the Senat der Freien und Hansestadt Hamburg as state secretary under Ole von Beust.

Until April 30, 2006, he was state secretary for health, before becoming state secretary in the newly formed Ministry of Social Affairs, Family Affairs, Health and Consumer Protection on May 1, 2006.

Wersich became acting Minister of Schools and Vocational Training as well as Deputy Mayor of Hamburg until he was replaced on March 7, 2011 by Dorothee Stapelfeldt of the Social Democratic Party (SPD) following the February 20 Hamburg state election.

On March 7, 2011, Wersich was elected chairman of the CDU parliamentary group in the Hamburg state legislature (Hamburgischer Bürgerschaft), thus becoming the leader of the largest opposition party.