After attending the Gymnasium at Rastenburg,[2] Haase studied law in Königsberg (now Kaliningrad, Russia), joined the Social Democratic Party of Germany (SPD) in 1887 and the next year established himself as a lawyer.

In 1907, Haase was counsel for Karl Liebknecht (SPD) who had been charged with high treason for publishing his pamphlet Militarismus und Antimilitarismus.

[3] Haase belonged to the so-called "revisionist" wing of the party, which in contrast to the Marxists, supported gradual reforms and no longer saw the best path to social and political change in revolution.

[3][1] In the course of the German Revolution in November 1918, along with the majority Social Democrats' leader Ebert, Haase became joint chairman of the provisional government, the Council of the People's Deputies.

[3] After the Council led by Ebert ordered the bloody suppression of the revolutionary Volksmarinedivision during Christmas 1918, Haase and the two other USPD representatives Wilhelm Dittmann and Emil Barth left the government on 29 December in protest.

[3] Even then, Haase supported continued cooperation with the SPD and was in favour of elections to the Weimar National Assembly—both views not universally popular in his party where many preferred a council-based republic.

[1] After the founding of the Kommunistische Partei Deutschlands (KPD) in late 1918/early 1919, Haase argued in favour of a reunification between USPD and majority SPD.

It also made their job of feeding the population, keeping up law and order and decommissioning the huge war-time army whilst replacing the Empire with a republic harder by threatening to antagonize the mostly conservative civil service and, in particular, the leadership of the military.

On 8 October 1919, immediately prior to his death Haase was walking into the Reichstag with the intention of exposing an alliance between Ebert and Rüdiger Von der Goltz, a Freikorps general active in the Baltic.