Madgearu was a prominent opponent of the National Liberal Party for much of his life, developing an original theory that challenged both Liberal tenets and Marxian economics, proposing measures to enhance the political and economic roles of Romanian peasants. In his later years, he was involved in anti-fascist causes, and was one of several politicians to be assassinated by the Iron Guard.
After being employed by an insurance company, Madgearu was teacher at the Academy for High Commercial and Industrial Studies in Bucharest after 1916, and held the position until his death. With Dimitrie Gusti, he founded the Romanian Social Institute, which aided sociologists in first-hand investigation work.
As one of the leaders in the parliamentary opposition to the Alexandru Averescu government, he was at the center of a scandal in July 1921: during a prolonged debate, he was addressed an insult by the People's Party politician Constantin Argetoianu; promptly, the National Liberal Party (PNL) leader Ion G. Duca expressed his sympathy, and helped weaken political support for Averescu (the cabinet was to fall in autumn of that year). Nevertheless, Madgearu's rejection of PNL policies and the Ion I. C. Brătianu government led him to join, in 1923, Liga Drepturilor Omului (the League for Human Rights), reuniting a diverse group of leftist activists such as Constantin Rădulescu-Motru, Constantin Mille, Nicolae L. Lupu, Constantin Costa-Foru, Constantin Titel Petrescu, Dem I. Dobrescu, Victor Eftimiu, Radu Rosetti, and Grigore Iunian; it was active until 1928.
He held several governmental positions in the Iuliu Maniu, Gheorghe Mironescu, and Alexandru Vaida-Voevod PNŢ cabinets: he was Minister for Industry and Trade (1928-1929; June-October 1930; August-October 1932), Finance Minister (1929-1930; 1932-1933), and Minister for Agriculture and Royal Domains in 1931. He also represented Romania at the League of Nations conferences on economy (in the context of the Great Depression), and was secretary of the PNŢ in 1926, as well as its leader for Ilfov County.
Madgearu was initially supportive of King Carol II, whom his party had helped bring to power, and remained sympathetic despite the confrontation between the monarch and the PNŢ leader Iuliu Maniu. According to the ironic account given by Petre Pandrea, the connection was tested by intrigue, after the writers Sergiu Dan and Ion Vinea allegedly stole a text by Madgearu (which was supposed to be published in Dreptatea), forged it by adding statements critical of Carol's policies, and sold it to Madgearu's main adversary inside the party, Mihail Manoilescu (Carol himself dismissed the letter as a fake, contributing to the ultimate PNŢ inner-conflict that caused Manoilescu to leave the group).
After Carol established his personal dictatorship, he continued to side with the PNŢ, which was active in semi-clandestinity. According to the PNŢ activist Ioan Hudiţă, Madgearu, with Ion Mihalache and Mihai Popovici, continued to support the king, and, after 1938, considered joining the National Renaissance Front.
An adversary of the fascist Iron Guard, he staunchly opposed its rise and the National Legionary State established in September 1940. Later in that year, after the remains of Corneliu Zelea Codreanu were discovered at Jilava (and the conclusion was drawn that he had been murdered on the orders of King Carol), Madgearu and Nicolae Iorga were among the victims of a wave of assassinations carried out in reprisal. Hours after the Jilava Massacre, Madgearu was attacked in his Bucharest house, kidnapped, and taken to the Snagov forest, where he was shot several times.
During the following decades, his killing remained a debated topic. It became apparent that Horia Sima (Codreanu's successor) was not able to justify it using the predilect excuse, as a direct answer to previous repressive measures. Unlike Iorga, who had assisted in Codreanu's arrest, Madgearu had not been responsible for any violent actions, and was probably targeted only for having served under Carol. Sima later argued that the series of murders had no relevant impact on public opinion (and even that it had led to an increase in his party's appeal). However, condemnation of the actions was widespread, and the resulting negative image probably contributed in rallying political forces behind traditional authorities, and the eventual ousting of the Guard by the Ion Antonescu-led Romanian Army (January 1941; see also Romania during World War II).
Madgearu's leftist views were treasured in retrospect by Nicolae Ceauşescu's communist regime, with its ethnocentric focus on regaining "progressive" precedents that were not actually communist (alongside Madgearu, several other Liga Drepturilor Omului members, and Iorga, the list also included Nicolae Titulescu, Traian Bratu, Grigore Filipescu, and Mitiţă Constantinescu).
Posthumously, in 1990, the Romanian Academy elected him a member.
In 1919, upon the creation of the Peasants' Party, Madgearu wrote:
"If the methods of various socialist parties differ, if the majority of the socialist world does not believe in the means of Russian Marxism, their common goal is the very same.
If, in a state where the majority of the population is comprised of industrial proletarians, the tendency of socialism to conquer political power may be viewed as a natural and justified development, in a state where the proletariat is a minority such efforts cannot ever correspond with the natural evolution of things.
Nevertheless, the Russian example shows that the possibility for a dictatorship of the proletariat, even in a country where [the proletariat] is manifestly numerically inferior, is not excluded if the largest segment of the population is amorphous and politically inert as was the case of the peasantry in the Muscovite Empire [sic].
Within its new borders [that is, those of Greater Romania], Romania endures as an agrarian-industrial state, in which the rural inhabitants shall form more than three quarters of the population. Thus, what then ought to be more natural and necessary today, after the Russian lesson, than the political rallying of the peasantry, [than] its transformation from an amorphous and politically inert mass into a self-aware organism that is to demand its share of stately power in proportion to its numbers and significance?
He believed that the development of the country was not to follow strict capitalist guidelines, and foreign capital was needed without neglecting the domestic one. Given that the export capacity of the country was very limited, he insisted on developing the domestic market. As the a labor force with medium qualification was missing in Romania, Madgearu called for the development of a proper training system which was to provide skilled professionals for the industry.
In this context, he, as much as the Peasants' Party leader Ion Mihalache, supported cooperative farming (with it, he primordially aimed to preserve the small-scale rural holding, which he saw as a viable economic cell for a relative future). Nevertheless, during the Great Depression, he revised some of his principles, and, like much of his party, advocated a focus on accelerated industrialization — as a means to preserve Romania's independence.
"If the peasant doctrine admits that the basis of its policies is class-focused, its concept of human society is not class-based and its ideas are less class-based than those of any other party. The other parties label themselves "socially harmonious", taking pride in themselves as national, fusing in their concerns the interests of all citizens. The peasant doctrine knows that they are, in reality, class-based parties, and, if it opposes them, it is precisely because of a national necessity, in order to ensure the normal social development of the people.
[...] The future society can only be a community of producers of manual and intellectual services, in which the labour of the one satisfying a human need, from bread production to poetry, shall be the only warrant for existence.
The economic and political doctrine of "socially harmonious" parties is domination of capital over labour. On the contrary, the peasant doctrine sees labour as dominating capital. The peasant should achieve a self-sufficient economic existence, and the industrial labourer should become an active collaborator in production, in cooperation with the intellectual labourer and the representatives of organized consumers.
His view on the role of peasant doctrine remained present in the National Peasants' Party program, as illustrated in Iuliu Maniu's 1926 speech on the group's ideology:
"If the National Peasants' Party on one hand relies on all the working and producing classes and aims to justly defend all these classes' economic and social interests, then, on the other, it cannot fail to note that the immense majority of the nation is formed by the peasant category.
"[...] in political rallying, peasant doctrine does not approve of the financial oligarchy's political domination, and strives to promote a truly democratic government, based on the freely-expressed will of popular masses, whose political awareness it seeks to awaken.
He expanded on this particular ideal in a 1924 article for Aurora:
"[If the Peasants' Party is to be victorious in elections] the shape of things would be changed. The National Bank would no longer be the economic fortress of the Liberal oligarchy. Trusts would no longer enslave and exploit the state. Their selfish and venal leaders would no longer be enthroned in overseeing positions over the country's destinies. Civil liberties nowadays suffocated and stolen civil rights would be fully restored, and the constitutional-parliamentary regime would become a reality, benefiting the development of popular masses as well as civilization.
Madgearu notably defended the vision of Constantin Dobrogeanu-Gherea in front of criticism from the maverick Liberal Ştefan Zeletin, who had written a comprehensive study of the Romanian bourgeoisie, overtly sympathetic towards economic liberalism.
"It could be objected, however, that peasant doctrine is hostile toward industry. It has been indicated that there is no disagreement between the development of an autonomous class of peasants and major industries, and that, quite the contrary, a mutual fulfillment was discovered between the development of peasant agriculture and major industrial enterprises...
In this sense, there is no question of agricultural policies versus industrial policies, of permanent and determined disagreement. Not even the doctrine of Poporanism denied the necessity of industrialization, but rather only the possibility of establishing a capitalist industry in underdeveloped agrarian environments. It is probable that the process of transformation of past agrarian states into industrial ones, on the basis of private property and free competition, will not be identically reproduced in present-day agrarian states. Even the social-democrat Kautsky stated that, in social transformation, there is no way of conceiving that an agricultural country should cover the same length and direction of the path taken by present-day industrial states.
"Without treasuring beyond measure the absolute value of planning in organizing the national economy, experience has shown the superiority of plan-based state intervention over that left to chance [...].
The multiple and varied interventions of the state in agriculture and industry, lacking any directive and continuity, are responsible for [the weakening of national economy].
[...] An economic plan does not at all imply the suppression or even a narrowing of private initiative in the economic life. An economic plan means containing and applying discipline to individual economic forces while maintaining the role of private initiative. An economic plan means coordinating the efforts of individual economic organizations and empowering them, through association and the systematic contribution of the state as the true representative of the national community.