The Lega Lombarda would later seek alliances with similar movements in Veneto and Piedmont, forming the Northern League, of which he was the federal secretary until 5 April 2012.

When the scandals of Tangentopoli were unveiled from 1992 on, Bossi rode the wave, presenting himself as the new man in politics, and set out to sweep away corruption and incompetence.

However, after suffering a stroke on 11 March 2004, which seriously impaired his speech, he quit on 19 July 2004 to take up a seat as a member of the European Parliament, where he registered an attendance to 9 per cent of the plenary sessions in his last mandate.

On 11 January 2005, Bossi appeared on the political scene at the last house of the Lombard federalist politician Carlo Cattaneo at Lugano after 306 days from the accident.

During that day, he met the Minister of Economy Giulio Tremonti (Forza Italia) with whom he constituted the political agreement called the "Alliance of the North" (Asse del Nord).

He also met a representative of the Lega dei Ticinesi, a Swiss localist Movement led by the Luganese entrepreneur Giuliano Bignasca.

In September 2007, Bossi accepted an invitation by Father Florian Abrahamowicz to his celebration of a Tridentine Mass and said there were affinities between the Lega Nord and the followers of Archbishop Marcel Lefebvre.

On 5 April 2012, when news broke of an alleged appropriation of party funds for the private affairs of his family, Umberto Bossi resigned as federal secretary of Northern League.

[17] With a decision of August 2019, the Supreme Court of Cassation decreed, as reported by Reuters, that "the case against former League leader Umberto Bossi and its former party treasurer had expired due to the statute of limitations, but the confiscation of the funds remained in place.