From April 2000 to December 2002, he was the president of the Youth of the party.[2] He also served as the spokesperson of the party from March 2001 to December 2002. He was an MP in the National Assembly of Serbia from January 2001 to December 2003. From 2004 to 2006 he was an MP in National Assembly of Serbia and Montenegro and several other European Boards.[1][2]
From 2008 to 2013 he served as the president of position parliamentary group of Socialist Party of Serbia in National Assembly, as well as a member of the permanent delegation of the National Assembly of Serbia to the Parliamentary Assembly of the Council of Europe, where he was a member of the Political Committee and a member of the Human Rights Committee. In September 2013, after the reconstruction of the cabinet of Ivica Dačić, he was named the Minister without portfolio in charge of European integration and stayed on that position until 27 April 2014.[2]
Ružić was named the Minister of Education and the First Deputy Prime Minister of Serbia when the new cabinet is formed on 28 October 2020. After the 2022 elections, he stopped being the First Deputy Prime Minister of Serbia, but still remained the Minister of Education.[2]
Following the Belgrade school shooting on 3 May 2023, Ružić received backlash after claiming that "the cancerous, pernicious influence of Internet video games, so-called Western values, is evident in the shooting".[8] Numerous opposition parties called for his resignation, including the Youth Initiative for Human Rights, and the Independent Union of Educators of Serbia.[9][10][11] He resigned from his position as minister of education on 7 May.[12] The National Assembly officially dismissed him on 29 May.[13] He was appointed advisor to Ivica Dačić in late May.[14]