Throughout most of his life, he was the chief ideological opponent of the Polish military and political leader Józef Piłsudski and of the latter's vision of Poland as a multinational federation against German and Russian imperialism.

A controversial personality most of his life, Dmowski desired a homogeneous, Polish-speaking and Roman Catholic-practicing nation as opposed to Piłsudski's vision of Prometheism, which sought a multi-ethnic Poland reminiscent of the Polish–Lithuanian Commonwealth.

As a result, his thinking marginalized other ethnic groups living in Poland, particularly those in the Kresy (which included Jews, Lithuanians, and Ukrainians), and he was regarded as anti-Semitic.

[6] After 1890 he was also developing as a writer and publicist, publishing political and literary criticism in Głos, where he became close friends with Jan Ludwik Popławski, who would be his mentor.

[8] Dmowski went to Jelgava, and soon afterward in early 1895 to Lemberg, Austria-Hungary (modern Lviv, Ukraine, Lwów in Polish), where together with Popławski he began to publish a new magazine, Przegląd Wszechpolski [pl] (All-Polish Review).

[7][11] In Myśli nowoczesnego Polaka, Dmowski was harshly critical of the old Polish–Lithuanian Commonwealth for exalting the nobility and for its tolerance for minorities, which contradicted his principle of healthy national egoism.

[13] He travelled to Tokyo to work out the details, and at the same time made a successful effort to prevent the Japanese from aiding a rival Polish political activist, Józef Piłsudski, who wanted assistance for a planned insurrection in Poland, an aspiration which Dmowski felt would be doomed to failure.

[14] During the revolt in Łódź in June 1905, the Endeks, acting under Dmowski's orders, opposed the uprising led by Piłsudski's Polish Socialist Party (PPS).

[14] As a result of the elections to the First Duma (legislative assembly in the late Russian Empire), which were boycotted by the PPS, the National Democrats won 34 of the 55 seats allotted to Poland.

[21][25] However, subsequent attempts on the part of Dmowski to have the Russians make firmer commitments along the lines of the Grand Duke Nicholas's manifesto were met with elusive answers.

[21][25] Nonetheless, Dmowski's pro-Russian and anti-German propaganda succeeded in frustrating Piłsudski's plans of causing an anti-Russian uprising, and bolstered his position as an important Polish political figure on the international scene, especially with the Triple Entente.

[22] In 1915, Dmowski, increasingly convinced of Russia's impending defeat, decided that to support the cause of Polish independence he should go abroad to campaign on behalf of Poland in the capitals of the western Allies.

[26][27] He was a vocal critic of Austro-Hungary, and campaigned for the creation of a number of Slavic states (including for the Czechs, as well as non-Slavic Hungarians and Romanians) in its place.

[29] Within the Polish political community, he opposed those who supported allying themselves with Germany and Austria-Hungary, including supporters of a vague German proposal for a Regency Kingdom of Poland, with undefined borders, that Germany promised to create after World War I (while in secret, actually planning to strip it of up to 30,000 square kilometres for German colonization after the removal of its Polish population).

The American President Woodrow Wilson reported, "I saw Mr. Dmowski and Mr. Paderewski in Washington, and I asked them to define Poland for me, as they understood it, and they presented me with a map in which they claimed a large part of the earth.

"[40] Another area of objection to Dmowski was with his antisemitic remarks, as in a speech he delivered at a dinner organized by the writer Gilbert Keith Chesterton, that began with the words, "My religion came from Jesus Christ, who was murdered by the Jews.

[41] In turn, Dmowski's experiences at that time convinced him of the existence of an international "Judeo-Masonic conspiracy, unfriendly towards Poland and intransigently hostile to his [Endecja] party".

To put an end to the rival claims of Piłsudski and Dmowski, the composer Ignacy Jan Paderewski met with both men and persuaded them to reluctantly join forces.

[26] On 29 January 1919, Dmowski met with the Allies' Supreme War Council for the first time; his five-hour presentation there, delivered in English and French, was described as brilliant.

[47] At the meeting, Dmowski stated that he had little interest in laying claim to areas of Ukraine and Lithuania that were formerly part of Poland, but no longer had a Polish majority.

[49] With regard to the former Austrian province of East Galicia, Dmowski claimed that the local Ukrainians were quite incapable of ruling themselves and also required the civilizing influence of Polish leadership.

[56][57] Dmowski regarded Piłsudski's views as dangerous nonsense, and felt that the presence of large number of ethnic minorities would undermine the security of Polish state.

[60] In 1926, in the aftermath of Piłsudski's May coup d'état, Dmowski founded the Camp of Great Poland (Obóz Wielkiej Polski), though he would find himself more of an ideologue than a leader, as he was displaced by new, younger politicians.

Władysław Jabłonowski, who spent the summer of 1936 with him in Tłokinia, noted that Dmowski "was neither eager to write, which he liked to do in the countryside, nor to go for long walks, which he was a great fan of; he preferred to sit for hours in the shade of trees."

[73] Dmowski felt very strongly that Poles should abandon what he considered to be foolish romantic nationalism and useless gestures of defiance and should instead work hard at becoming businessmen and scientists.

[77]In the pre-war years, the history of Poland was contested terrain as different ideological forces pulled Polish nationalists in opposite directions, represented by Dmowski and Piłsudski.

[66] He opposed physical violence,[23][90] arguing for the boycotts of Jewish businesses instead,[91] later supplemented with their separation in the cultural area (through policies such as numerus clausus).

[93][94] In his 1938 essay Hitleryzm a Źydzi, Dmowski wrote: "The tool of the Jews was Wilson, who was concerned that the Allied troops did not cross the German border...Lloyd George stopped regions from becoming part of Poland as they were before: the great majority of our Upper Silesia, Malborg, Sztum and Kwidzyn, and also Gdansk.

Dmowski was never able to have this program passed into law by the Sejm, but the National Democrats did frequently organize "Buy Polish" boycott campaigns against German and Jewish shops.

[98] Dmowski is considered one of the most influential conservative politicians in the history of modern Poland, although his legacy is controversial and he continues to be a highly polarizing figure.