Guerini began his political career at the beginning of the 1990s in the ranks of the Christian Democracy. He was elected twice as municipal councilor in Lodi for the DC, where he also carried out the functions of councilor for social services. In 1994 he was chosen as the local coordinator of the new-born Italian People's Party, the direct heir of DC. In May 1995, he was elected first president of the Province of Lodi representing the centre-left coalition and becoming the youngest provincial president in Italy at the age of 28. In June 1999 he won a second term, during which, following the dissolution of the Italian People's Party, he The Daisy, a Christian leftistpolitical party, of which he became a main national leader. In the 2005 local elections, Guerini was elected Mayor of Lodi for the centre-left, obtaining 54.1% of valid votes in the first round; he was also appointed president of the Lombardy section of the National Association of Italian Municipalities between 2005 and 2010. Together with the majority of The Daisy members, in 2007 he joined the Democratic Party, the new centre-left party, born from the union between The Daisy and the Democrats of the Left. In the 2010 local elections, Guerini was again elected mayor in the first round of voting, with 53.7% of the votes. During his term as mayor he enjoyed a broad and widespread popular consensus, becoming one of the most appreciated mayors in Italy. On 31 December 2012, Guerini resigned from his post, in order to present his candidacy for the upcoming parliamentary election in February 2013, in compliance with the eligibility conditions.
National leader of the PD
After winning the special primaries organized by the Democratic Party, Guerini was included in the list of candidates for the Lombardy 3 constituency of the Chamber of Deputies and was finally elected. In 2013, Guerini endorsed Matteo Renzi in the 2013 leadership election, who appointed him, member of the national secretariat of the party, with the role of spokesman. In February 2014 he joined the newly appointed Prime Minister Renzi in carrying out institutional consultations for the formation of the new government. On 14 June 2014, the PD national assembly approved Guerini's appointment as party's Deputy Secretary, along with Debora Serracchiani. Following the 2017 leadership election, they both ceased from office on 7 May 2017, replaced by Maurizio Martina. However, Guerini held a position within the party leadership, becoming coordinator of the national secretariat. In the 2018 general election Guerini was re-elected in the Chamber of Deputies and on 18 July 2018 he was elected president of Parliamentary Committee for the Intelligence and Security Services and for State Secret Control, a body of the Italian Parliament deputed to survey and oversee the activities of the Italian intelligence agencies.
Minister of Defence, 2019–present
In August 2019 tensions grew within the populist government, leading to the issuing of a motion of no-confidence on Prime Minister Giuseppe Conte by the League. After Conte's resignation, the national board of the PD officially opened to the possibility of forming a new cabinet in a coalition with the M5S, based on pro-Europeanism, green economy, sustainable development, fight against economic inequality and a new immigration policy. The party also accepted that Conte may continue at the head of a new government, and on 29 August President Sergio Mattarella formally invested Conte to do so. On 5 September, Guerini was appointed new Minister of Defence. In response to the COVID-19 pandemic in Italy, Guerini has said that there will be an extraordinary enrollment of doctors.
