The National Union (Portuguese: União Nacional, UN) was the sole legal party of the Estado Novo regime in Portugal, founded in July 1930 and dominated by António de Oliveira Salazar during most of its existence. Unlike most single-party regimes, the National Union was more of a political arm of the government than a body with actual power over it. The National Union membership was mostly drawn from local notables: landowners, professionals, businessmen, Catholics, monarchists, or conservative republicans. The National Union was never a militant or very active organization.

Once Salazar assumed the premiership, the National Union became the only party legally allowed to function under the Estado Novo. Salazar announced that the National Union would be the antithesis of a political party. The NU became an ancillary body, not a source of political power. At no stage did it appear that Salazar wished it to fulfill the central role the fascist party had acquired in Mussolini's Italy; in fact, it was meant to be a platform of conservatism, not a revolutionary vanguard.

The National Union's ideology was corporatism, and it drew inspiration from Catholic encyclicals such as Rerum novarum and Quadragesimo anno as well as from Mussolini's corporate state. Unlike fascist parties, the National Union played no role in the government: it only served as a tool for the selection of National Assembly deputies, as well as a way to provide some legitimacy to non-competitive elections that Salazar's regime regularly held. The National Union was set up to control and restrain public opinion rather than to mobilize it, and ministers, diplomats and civil servants were never compelled to join the party.

According to António Costa Pinto, the National Union was a moribund party, created by a governmental decree rather than by political activists, and which was "dominated by the administration, put to sleep and reawakened in accordance with the situation at the time". He describes the party as "an empty, undermined space into which were formally sent those who wanted to join the regime and which, once full, was closed". Pinto notes that the army was kept away from public life, and political activity was prohibited outside public life. This included the National Union, which lacked any kind of political activism.

Therefore, the party lacked an ideology and did not mobilize the masses. Pinto argues that the opposite was the case: "in fact demotivation was openly encouraged." He concludes that the party had a "non-fascist nature" and argues that it "neither reached power at all nor, once created, fulfilled functions of control and monopoly of access to power or mobilization of the masses, which, in general, the fascists did."

Scholarly opinion varies on whether the Estado Novo and the National Union should be considered fascist. Salazar himself criticized the "exaltation of youth, the cult of force through direct action, the principle of the superiority of state political power in social life, [and] the propensity for organizing masses behind a single leader" as fundamental differences between fascism and the Catholic corporatism of the Estado Novo. Scholars such as Stanley G. Payne, Thomas Gerard Gallagher, Juan José Linz, António Costa Pinto, Roger Griffin, Grzegorz Rossoliński-Liebe^{[citation needed]} and Arnd Bauerkämper [de], as well as Howard J. Wiarda, consider the Portuguese Estado Novo conservative authoritarian and not fascist.^{[citation needed]} In his The Anatomy of Fascism, Robert Paxton express the same view, writing that Salazar's regime was "not only nonfascist, but voluntarily nontotalitarian". On the other hand, Portuguese scholars like Fernando Rosas, Manuel Villaverde Cabral, Manuel de Lucena, Manuel Loff and Raquel Varela think that the Estado Novo should be considered fascist.

Salazar's political philosophy formed the ideological foundation of the Estado Novo and, by extension, defined the National Union's institutional role as a supporting organ rather than an autonomous ideological movement. Deeply influenced by Catholic social doctrine, particularly the papal encyclicals Rerum novarum (1891) and Quadragesimo anno (1931), Salazar advocated a corporatist and hierarchical social order intended to harmonize relations between social groups while avoiding class conflict. This system rejected both liberal capitalism and socialism, instead promoting cooperation between occupational and social bodies within a unified national framework.

Central to this worldview was the concept of integral nationalism, influenced by Portuguese integralist thought and the ideas of French thinker, Charles Maurras, which emphasized national unity, hierarchy, tradition, and opposition to liberal democracy. The regime rejected liberal parliamentarism, political pluralism, and socialism, advocating instead an organic conception of society in which individuals were represented through corporative institutions rather than political parties. These principles reflected broader intellectual influences from movements such as Integralismo Lusitano, which combined nationalism, traditionalism, monarchism, and anti-liberalism.