Sejm of the Second Polish Republic (1919 -- 1939) | Los Angeles Museum of the Holocaust

After the First World War and re-establishment of Polish independence, the convocation of parliament, under the democratic electoral law of 1918, became an enduring symbol of the new state's wish to demonstrate and establish continuity with the 300 year Polish parliamentary traditions established before the time of the partitions. Maciej Rataj emphatically paid tribute to this with the phrase: "There is Poland there, and so is the Sejm".

Józef Beck, Minister of Foreign Affairs, delivers his famous Honor Speech in the Sejm, 5th May 1939.

During the interwar period of Poland's independence, the first Legislative Sejm of 1919, a Constituent Assembly, passed the Small Constitution of 1919, which introduced a parliamentary republic and proclaimed the principle of the Sejm’s sovereignty. This was then strengthened, in 1921, by the March Constitution, one of the most democratic European constitutions enacted after the end of the First World War. The constitution established a political system which was based on Montesquieu's doctrine of separation of powers, and which restored the bicameral Sejm consisting of a lower house (to which alone the name of "Sejm" was from then on applied) and an upper house, the Senate. In 1919, Roza Pomerantz-Meltzer, a member of the Zionist party, became the first woman ever elected to the Sejm.

The legal content of the March Constitution allowed for Sejm supremacy in the system of state institutions at the expense of the executive powers, thus creating a parliamentary republic out of the Polish state. An attempt to strengthen executive powers in 1926 (through the August Amendment) proved too limited and largely failed in helping avoid legislative grid-lock which had ensued as a result of too-great parliamentary power in a state which had numerous diametrically-opposed political parties sitting in its legislature. In 1935, the parliamentary republic was weakened further when, by way of, Józef Piłsudski's May Coup, the president was forced to sign the April Constitution of 1935, an act through which the head of state assumed the dominant position in legislating for the state and the Senate increased its power at the expense of the Sejm.

On 2 September 1939, the Sejm held its final pre-war session, during which it declared Poland's readiness to defend itself against invading German forces. On 2 November 1939, the President dissolved the Sejm and the Senate, which were then, according to plan, to resume their activity within two months after the end of the Second World War; this, however, never happened. During wartime, the National Council (1939–1945) was established to represent the legislature as part of the Polish Government in Exile. Meanwhile, in Nazi-occupied Poland, the Council of National Unity was set up; this body functioned from 1944 to 1945 as the parliament of the Polish Underground State. With the cessation of hostilities in 1945, and subsequent rise to power of the Communist-backed Provisional Government of National Unity, the Second Polish Republic legally ceased to exist.