[citation needed] In the state elections in 1999, the CDU began collecting signatures to document the resistance in the population to plans of the federal government to make dual citizenship easier for foreigners to obtain.

[4] In 2003, Koch and Peer Steinbrück, the Social Democrat premier of North Rhine Westphalia, together drew up a plan to reduce tax breaks and subsidies, including those on coal by 12 percent over several years.

[5] Ahead of the 2004 German presidential election, Koch publicly endorsed Wolfgang Schäuble as the Christian Democrats’s candidate to succeed incumbent President Johannes Rau.

[6] Under the leadership of party chairwoman Angela Merkel, Koch was elected vice-chairman of the CDU in November 2006, alongside Jürgen Rüttgers, Annette Schavan and Christian Wulff.

[7] By 2007, he and Rüttgers, his counterpart from the state of North Rhine-Westphalia, agreed on approving a merger of their respective state-owned banks, WestLB and Landesbank Hessen-Thüringen (Helaba).

Prior to the Hesse state election of 2008 Koch was once again accused of using xenophobic tactics by pledging to get tough on youth crime, which is concentrated among immigrant and minority groups.

In the course of the election he lost his party's majority in the Hessian Parliament, but remained acting Minister President as his SPD challenger Andrea Ypsilanti was unable to form a government.

In the January 2009 election the FDP made significant gains which allowed Koch to form a conservative-liberal coalition government, reelecting him as the Minister President of the State of Hesse.

[19] Ahead of the Christian Democrats’ leadership election in 2018, Koch publicly endorsed Friedrich Merz to succeed Angela Merkel as the party's chair.

[23] During his tenure, Bilfinger Berger also agreed with the United States Department of Justice in 2013 to pay $32 million to resolve U.S. criminal charges that it bribed Nigerian officials to obtain contracts on a gas project in the African nation.