The Matteotti Affair:

Some WWW references:

When, in 1924, Mussolini was believed to have had the most respected leader of the Socialists, Matteotti, removed by murder -- his public utterances on the murder were so gross and callous that his guilt seemed clear -- so many turned against him that at the elections of 1926 his power was ominously shaken. Joseph McCabe

On October 28, 1922, after the Fascists had marched on Rome, Mussolini secured a mandate from King Victor Emmanuel III (1869-1947) to form a coalition government. In 1925-26, after a lengthy crisis with parliament following the assassination of the Socialist leader Giacomo Matteotti (1885-1924), he imposed a single-party, totalitarian dictatorship. His corporative state came to terms with Italian capitalism but abolished free trade unions. The History Guide

In July 1923, Mussolini was able to secure a new electoral law from the parliament. The new law provided that any party, having 25% of the votes in a general election, should receive two-thirds of the seats in the Chamber of Deputies. Mussolini immediately arranged for elections to the Chamber of Deputies in April 1924. In an atmosphere of intimidation and violence, with the Fascist Militia using strong-arm methods, the 'National List' presented by the Fascists obtained 63% of the roses. In June 1924 when the new Parliament convened, the Socialist leader, Giacomo Matteotti, denounced the Fascists of the use of force in the recent elections. He was immediately murdered by the Fascists.
Matteotti's murder led to an outcry against Mussolini. The parties in opposition to Mussolini's government withdrew from the parliament. This was called the Aventine Secession. The Aventine Secession only strengthened Mussolini's determination to use force to wipe out all his opponents. Fascist Italy