In the 2001 elections he was elected to the Senate of Italy in the first-past-the-post constituency of Albino. He then became the vice-president of the Senate until July 2004, when he was appointed Minister for Institutional Reforms in the place of Umberto Bossi, the longtime leader of the Northern League who had suffered a serious stroke and could not perform his duties. During his mandate, he also wrote a new electoral law based on proportional representation with a strong majority premium rather than plurality voting system, which was first introduced in Italy in 1994 by a referendum. Successively, Calderoli himself criticized the electoral law he wrote by defining it "una porcata" (literally, "a piggish stuff").