Rossi was a member of the Peronist University Youth in the 1970s, and joined the Justicialist Party in the 1980s. He elected to the Rosario Deliberative Council in 1987, but returned to his private practice in the 1991. He returned to politics in 2001, was elected to the Deliberative Council in 2002, and presided said body from 2004 to 2005. In the 2005 legislative elections he ran for a seat in the Lower House of Congress for the Front for Victory, the largest faction of the Justicialist Party, formed by supporters of PresidentNéstor Kirchner. His campaign capitalized on, and was centered on gaining support for, the successful policies implemented by Kirchner on the national level; Rossi, who was not yet well known in the province, presented himself as "Kirchner's candidate." The Front for Victory party list headed by Rossi came in second after the SocialistParty list headed by Hermes Binner.
He then became the head of the Front for Victory caucus in the lower house of Congress. In that capacity, he took on the task of imposing party discipline while rallying legislative support of Kirchner's policies; he stated that "the caucus has the fundamental role of being the legislative arm of the government. I cannot conceive a block that has doubts or criticism for the government's projects. Society tells us: 'I choose you to strengthen the course of action initiated by the President." He also courted controversy with his rhetoric, notably in August 2010 when he described lawmakers who presented a bill that sought reverse a government decision to revoke an expired license for the Fibertelinternet service provider, as "lawyers" for the Clarín Group ; there have been ongoing controversies between Clarín and Kirchnerism since 2008 over a number of issues. Rossi was a candidate in the FpV primary for the governorship of Santa Fe in the 2007 elections. His rival within the party was Rosario-born National Deputy and former Foreign Relations Minister Rafael Bielsa. The Mayor of Rafaela, Omar Perotti, campaigned for some time as well, but finally gave up and turned to support Bielsa. In the classical caudillist fashion that has prevailed in Argentina’s last century of politics, although some in the party, including Bielsa, would have rather negotiated on a consensus candidacy, Rossi insisted on conducting a primary election. Rossi also stated that he wanted Bielsa's sister María Eugenia, on his party list, though he instead chose Jorge Fernández, former Minister of Education during the governorship of Víctor Reviglio. Bielsa was eventually chosen as the party's candidate in primary elections on July 1, 2007, but subsequently lost the election to Socialist Hermes Binner. Rossi continued in his role as Majority Leader of the Chamber of Deputies as head of the Front for Victory caucus, which retained a majority in the Chamber. President Cristina Fernández de Kirchner nominated him to the post of Minister of Defense of Argentina on May 30, 2013. His tenure was overshadowed by suspicion of corruption and the theft of arms and ammunition - including a mid-range missile.
