At that time Bos was the leader of the largest opposition party. He spent considerable time reforming the PvdA's internal organisation and public image, together with the party's chairman Ruud Koole. He was criticized for his silence on important reforms and issues, and his moderacy which resulted in other parties claiming opposition leadership, among them most notably the Socialist Party. Although still enjoying large public support, criticism about his "style over substance" approach to politics was voiced in this period. At the December 2005 party congress, Bos announced that he aspired to become Prime Minister of the Netherlands, should the PvdA succeed in becoming the biggest party after the next parliament elections. And in the 2006 municipal elections, a few months later, the PvdA performed particularly well, becoming the largest party in local government.