Daszyński campaigned in the first post-war elections to the Polish Sejm, proclaiming: "The first legislative Sejm is the first administrator of Poland, its builder, the source of law and authority [in] a free, independent and united Poland."
36 members of the PPSD and PPS entered the Sejm and created a parliamentary group named Związek Polskich Posłów Socjalistycznych ("Union of Polish Socialist MPs").
Daszyński became its president.

He focused on promoting a socialist program.
He advocated nationalizing some industrial sectors, for example creating state monopolies in coal and spirits.
He proposed improvements in working conditions and protected workers' rights, as well as supporting development of the cooperative movement and education of peasants and workers.

On 26 April 1919, the PPSD, the PPS and the PPS Prussian Section united to form a unitary PPS.
Daszyński joined its General Council and became one of its chairmen.
He also edited its French-language publication Bulletin Official du Parti Socialiste Polonaise and the weekly magazine Trybuna.
After the outbreak of the Polish-Soviet War, Daszyński was for concluding peace as fast as possible.
He was opposed to the creation of the Council of National Defense, calling it an "abbreviation of the Sejm".
However, on 24 July he joined the Government of National Defense (Rząd Obrony Narodowej) as Deputy Prime Minister (Witos was Prime Minister).
He thought that this step (including peasant and socialist leaders) would increase the number of recruits.
After victory in the Battle of the Niemen River, Daszyński was more and more in conflict with the rest of the government, especially with the Ministry of Foreign Affairs.
He criticized diplomatic staff and Polish policy towards the East, in particular Tadeusz Rozwadowski's plans for an offensive.
On 15 December, the PPS General Council asked the Prime Minister to dismiss Daszyński, but he resigned himself on 18 December.
The Prime Minister accepted it very unwillingly, delaying till 4 January 1921.

After leaving government, Daszyński concentrated on working for the adoption of a new constitution.
On 17 March 1921, the Sejm adopted the March Constitution of Poland, after which it dissolved itself.
The socialist leader contributed to the democratic character of the constitution, for example by resisting the proposal of the conservatives that members could be appointed to the Polish Senate according to their position, without being elected.
Daszyński campaigned in the first post-war elections to the Polish Sejm, proclaiming: "The first legislative Sejm is the first administrator of Poland, its builder, the source of law and authority [in] a free, independent and united Poland." 36 members of the PPSD and PPS entered the Sejm and created a parliamentary group named Związek Polskich Posłów Socjalistycznych ("Union of Polish Socialist MPs"). Daszyński became its president.
He focused on promoting a socialist program. He advocated nationalizing some industrial sectors, for example creating state monopolies in coal and spirits. He proposed improvements in working conditions and protected workers' rights, as well as supporting development of the cooperative movement and education of peasants and workers.
On 26 April 1919, the PPSD, the PPS and the PPS Prussian Section united to form a unitary PPS. Daszyński joined its General Council and became one of its chairmen. He also edited its French-language publication Bulletin Official du Parti Socialiste Polonaise and the weekly magazine Trybuna. After the outbreak of the Polish-Soviet War, Daszyński was for concluding peace as fast as possible. He was opposed to the creation of the Council of National Defense, calling it an "abbreviation of the Sejm". However, on 24 July he joined the Government of National Defense (Rząd Obrony Narodowej) as Deputy Prime Minister (Witos was Prime Minister). He thought that this step (including peasant and socialist leaders) would increase the number of recruits. After victory in the Battle of the Niemen River, Daszyński was more and more in conflict with the rest of the government, especially with the Ministry of Foreign Affairs. He criticized diplomatic staff and Polish policy towards the East, in particular Tadeusz Rozwadowski's plans for an offensive. On 15 December, the PPS General Council asked the Prime Minister to dismiss Daszyński, but he resigned himself on 18 December. The Prime Minister accepted it very unwillingly, delaying till 4 January 1921.
After leaving government, Daszyński concentrated on working for the adoption of a new constitution. On 17 March 1921, the Sejm adopted the March Constitution of Poland, after which it dissolved itself. The socialist leader contributed to the democratic character of the constitution, for example by resisting the proposal of the conservatives that members could be appointed to the Polish Senate according to their position, without being elected.
Ignacy Daszyński