In May 2006 he was chosen as the SPD candidate for the Hanover mayoral election on 10 September 2006 against the CDU politician Dirk Topeffer and Ingrid Wagemann of Alliance '90/The Greens. He won an absolute majority in the first round. He succeeded Herbert Schmalstieg, the mayor of Hanover for 34 years on 1 November 2006. Weil held the office for 7 years, up to 2013 state election. Due to legal restrictions, Weil was automatically removed from the office of mayor when he became Minister President of Lower Saxony on 19 February 2013. From 29 January 2008 to 2011, Weil monthly answered questions from citizens in the TV program Warum Herr Weil which airs every third Tuesday every month on HR Fernsehen. On 18 September 2011 Weil announced that he would apply for the top candidate of the SPD for the 2013 state election in Lower Saxony. He was elected as the top candidate with 53.3% of votes on 27 September 2011. On 20 January 2012 he was voted as the chairman of SPD Lower Saxony. In March, Weil was unanimously chosen as the SPD direct candidate for the Hanover-Buchholz constituency. On the state convention in Hameln, Weil placed first with 98.95%.
Just weeks before the state elections, opinion polls indicated that Weil, with the help of the Greens, would easily defeat incumbent Minister-PresidentDavid McAllister. He eventually won the elections by a wafer-thin majority, resulting in his center-left coalition having a narrow majority of just one vote in the state parliament. At the time, his victory constituted the twelfth consecutive setback in a state vote for Chancellor Angela Merkel’s CDU party and therefore was widely interpreted as indicative for the national elections later that year. Early on in his tenure, Weil emphasized consolidating Lower Saxony's finances. As Lower Saxony has a 20 percent stake in Volkswagen, Weil has been an ex-officio member of the company's supervisory board since February 2013. Within the supervisory board, he serves on the mediation and the nomination committees. Only a few months after Weil took office, Germany won a decisive victory over the European Commission in its bid to preserve state influence at VW, when the European Court of Justice rejected an attempt by the commission to abolish a state veto over key decisions such as factory closures, mergers and acquisitions. In August 2017, Weil called for parliament to be dissolved and new elections to be held after one deputy, Elke Twesten, quit the Green Party, costing his coalition government its one-seat parliamentary majority. In November 2017, he was again elected Minister President with the votes of SPD and CDU.
Role in national politics
In his capacity as Minister-President, Weil was elected vice president of the Bundesrat from 1 March 2013, and served as President of the Bundesrat from November 2013 to October 2014. On the Bundesrat, he is a member of the Committee on Foreign Affairs and deputy chairman of the Committee on European Affairs. In the negotiations to form a Grand Coalition of Chancellor Angela Merkel's Christian Democrats and the SPD following the 2013 federal elections, Weil was part of the SPD delegation in the working group on energy policy, led by Peter Altmaier and Hannelore Kraft.
In 1987, Weil married public health expert Rosemarie Kerkow-Weil,, the former president of Leibniz University Hannover who teaches at the HAWK Hochschule Hildesheim/Holzminden/Göttingen, and they have one son.
