Already in 1919 Sleževičius had prepared the groundwork for the parliamentary elections. In April 1920, he was elected as the representative of II (Kaunas) constituency to the Constituent Assembly of Lithuania, tasked with drafting the Constitution of Lithuania. His Lithuanian Popular Socialist Democratic Party (LSLDP) formed a block with the Peasant Union, another breakaway from LDP. Later in 1920, after an armed conflict between Poland and Lithuania erupted, Sleževičius was elected the chairman of the Lithuanian Defense Committee ("Vyriausias Lietuvos gynimo komitetas"), tasked with organizing and supporting the armed forces. Even without a position on the government, Sleževičius remained an influential politician. According to prominent inter-war lawyer Michał Pius Römer, in 1921 Sleževičius was "one of the most powerful men in the country".