After attending the Gymnasium at Rastenburg, Haase studied law in Königsberg (now Kaliningrad), joined the Social Democratic Party of Germany (SPD) in 1887 and the next year established himself as a lawyer. He was the first socialist lawyer in East Prussia and mainly took on as clients people from the lower classes (workers, peasants), journalists and socialist functionaries.
In 1894 he became the first SPD deputy elected to Königsberg s city parliament, and in 1897-1906 and 1912-1919 he served his party in the Reichstag. A soft-spoken man devoid of personal ambitions and histrionics, Haase in 1911 succeeded Paul Singer as deputy leader of the party and two years later, after the death of August Bebel, joined Friedrich Ebert as cochairman of the SPD. Haase continued to nourish a deep sense of justice and right and from his office in Berlin became a renowned defender of workers in court, never charging for his legal services. Although he mastered Marxist rhetoric, Haase in spirit remained closer to the humanism and cosmopolitanism of his fellow Königsberger, Immanuel Kant.
Unlike Ebert and Philipp Scheidemann, Haase did not rise through the party bureaucracy; rather, he remained an outsider, a left-wing intellectual. Haase spoke out against militarism and imperialism and in favor of pacifism. On the eve of the Great War he opposed the demands of South Ger¬man (Baden) Socialists that the SPD cooperate with middle-class parties in the Reichstag.
August 1914 proved a bitter experience for Haase. During the July crisis he stood almost alone in denouncing the approach of war, and his public utterances raised for the government the prospect that the SPD might not vote for war credits. During the decisive party caucus on August 3, however, only 12 of the HO SPD deputies joined Haase in opposing the war; as caucus leader he was forced by party discipline to bow to the will of the majority and next day in the Reichstag to read his party's unanimous vote on behalf of war credits. In his heart, however, Haase knew that the decision had been wrong. On June 19, 1915, he joined Karl Kautsky and Eduard Bernstein in a famous Demand of the Hour, which denounced the government's annexationist war aims program. In March 1916, Haase formed a Social Democratic Working Union and formally voted against war credits; as a result, he was forced to relinquish coleadership of the SPD. Although suffering from frayed nerves and bouts of depression, Haase continued to campaign against censorship, the state of siege, and the lack of political and social equality in imperial Germany. Easter 1917 brought the final split in the party when Haase was elected head of a new Independent Social Democratic party (USPD) at Gotha the very place where over a half century earlier the various factions of German socialism had united. In place of the victor's peace demanded especially by the vociferous Pan-Germans, the USPD offered the notion of a peace without victors or vanquished.
Haase proved his vision and statesmanship during the revolution of November 1918. Curbing the romantic revolutionaries within the USPD, he counseled cooperation with Ebert's SPD in directing Germany towards a democratic, republican future. Haase joined Ebert on November 10 as coleader of the Council of People's Commissars, and one month later, during the First German Congress of Workers' and Soldiers' Councils, opted against the dictatorship of the proletariat in favor of a constituent national assembly, reform of the officer corps, and nationalization of heavy industry. Unfortunately, Ebert preferred to cling to his secret pact with General Wilhelm Groener in which the head of the Ger¬man labor movement had promised not to reform the aristocratic Prussian officer corps. And when at the end of the year Ebert ordered army units to dislodge radical sailors from the former royal stables in Berlin, Haase protested the blood spilled by resigning from the Council of People's Commissars.
Yet the level-headed Haase refused to bolt to the extreme left. Despite the bloody suppression of the Spartacist uprising in January 1919 and the cowardly murders of Rosa Luxemburg and Karl Liebknecht, Haase in March 1919 urged the USPD to support the SPD. His pleas on behalf of a "unified Socialist party" excluding the newly founded Communist party were to remain stillborn for him: Haase died on November 7, 1919, four weeks after a psychotic shot him with a revolver.
He joined the Social Democratic Party of Germany (SPD) in 1887.
Hugo Haase had been married to Thea (née Lichtenstein) with whom he had a son.