Carstensen was the CDU's candidate for premier of Schleswig-Holstein in the state election of 20 February 2005; he led the CDU to its best result since Uwe Barschel’s resignation in 1987 with 40.2% of all votes cast, making the CDU the strongest party in the state parliament for the first time since 1983. However, the coalition of the CDU and the Free Democratic Party (FDP) failed to obtain a majority by some 700 votes. The Social Democrat (SPD) office-holder Heide Simonis thus tried to form a minority cabinet of the SPD and Bündnis '90/Die Grünen (the Green party) tolerated by the Danish minority party SSW. From 1 March to 27 April 2005 Carstensen was leader of the CDU in the state parliament.