Regarded as an implacable opponent of communism, he was selected for the role (without his knowledge) by the leader of the PSI's right-wing current, Giuseppe Saragat, and was accepted as a compromise candidate by the other leading factions at the party's annual spring conference in Florence.

Lombardo was in Washington, D.C. as part of an official trade delegation when the decision was announced (in his capacity as Under-Secretary of Commerce and Industry in De Gasperi's first government), and he found out about it – with much incredulity and bemusement – only after reading reports on the conference in the American press and upon receiving congratulatory telegrams from well-wishers.

At the general election in April of that year he and the UdS allied with Saragat's PSLI to form a joint ticket under the banner of Socialist Unity (Unità Socialista), which gained 7.1% of the votes cast for the Italian Chamber of Deputies and 33 seats.

[6] Lombardo later argued that his principal reason for leaving the PSI was its opposition to the Marshall Plan, as he explained to Philip C. Brooks of the Harry S. Truman Library in 1964: Since the [PSI] Convention had decided to join the Communists in the Popular Front, and feature as the fundamental issue for the incoming general elections a staunch opposition to the Marshall Plan, I had no choice but to quit the party, and I formed a group which participated in the election campaign on the same ballot with Saragat's Social Democratic Party [sic]... For many years a large section of the Italian public opinion, which was heavily influenced by Communists, was convinced that the intent of the Marshall Plan was to destroy the industrial and agricultural structure of our society.

Two years later, in 1955, he was appointed as President of the General Committee and of the Board of the National Council for Productivity (CNP) in Rome, and in 1959–60 he served as head of the Atlantic Treaty Association (ATA) in Brussels.

This manifesto soon evolved into a new centrist political party led by Pacciardi, the Democratic Union for the New Republic (Unione Democratica per la Nuova Repubblica; UDNR), which made very little electoral impact but was noted for its association with figures from the neo-fascist right, such as Enzo Maria Dantini, Fabio De Felice and Giano Accame, who were attracted to the party by its emphasis on strong, personalised leadership and promises to "remake the state".

[13] This conference, which was funded through the Institute by the Italian military intelligence agency SIFAR, has since come to be regarded as a foundational moment in the strategia della tenzione ("strategy of tension"), and was attended by several individuals who were later involved in various neo-fascist terror campaigns.