Great question!

The interwar period was a challenging time for Jews in many of the new ethnostates formed out of the ruins of the prewar empires. While Jews from Europe and the US (on behalf of European Jews) lobbied heavily around the Treaty of Versailles, they were, in general, in a complex place, as while they were generally not seen as part of the ethnic majority, they also were not seen as their own ethnic/national minority, which had real implications especially in this age of nationalism. Many Jews sought to establish themselves as a national minority, with the various rights that would have come with this status; however, not all Jews did.

The specific agreement signed by the Polish government, in 1919, was the Minorities' Treaty. It specified that Jews would receive publicly funded schools and would not be compelled to work on the Sabbath, but not much else. While Jews were seen as a recognized national minority under the treaty (which they saw as a win), they didn't get approval for their own independent governing body, for proportional representation in the Sejm, etc- all things that they'd wanted. The Poles were angry that they felt compelled by foreign interference to sign the treaty. Besides the treaty, Jews were also protected by the 1921 constitution, which granted equality to all Polish citizens regardless of nationality or religion- making it unclear which category included Jews.

It should be noted that, in general, interwar Poland took on an effort of Polonization of all minorities, whether German, Lithuanian, Ruthenian/Belorussian... in this regard, Jews were not unique. However, antisemitism absolutely became its own factor. The fact that Jews weren't clearly an ethnic/national group made the question of how to conceptualize them complicated, as was the fact that they lived throughout Poland rather than in the borderlands; and of course the fact that Jews were not Christian, unlike the rest of the major minority groups, was very significant as well. Antisemitism had preexisting deep roots in Poland, as should be unsurprising.

Throughout the interwar period there were two dominant political parties- Endek (National Democrats), a right-wing party led by Roman Dmowski, and PPS, a(n initially) left-wing party led by Josef Pilsudski. Neither of these parties supported Jewish nationalism. Endek and Dmowski were, for the most part, outwardly and inwardly antisemitic. Dmowski claimed that Jews were enemies of Poland who were in league with Germany, wanted to rule the world, and had usurped the Paris peace accords. To Dmowski and others sharing his views, which included not only members of Endek but of center-aligned parties as well, while the idea of Jews assimilating was anathema (and seen as impossible), the idea of them forming their own nation to (in their view) destroy Poland from within was untenable as well. Their only real proposed solution was to attempt to rid Poland of its 3 million plus Jews. While in the 1930s this was done specifically through efforts to openly encourage them to emigrate (as was the case in Nazi Germany at this time as well), in the early days of interwar Poland, it often meant attempting to weaken the Jewish community by making their lives as difficult as possible. Among other things, this meant that parts of the Minorities' Treaty were abandoned (Jewish schools were not state subsidized, and graduates could not attend Polish universities) and Jewish national, rather than merely religious, autonomy was made impossible. Even worse, Jews were denied the equality under the law guaranteed in the constitution, and in fact were often actively persecuted by government bodies. Pogroms that occurred in the immediate aftermath of WWI included soldier involvement with no government consequences. Jews were nearly barred from government jobs, whether municipal, educational, medical, scholarly, etc. Blue laws banned work on Sunday, forcing Jews to abstain from work two days a week rather than just one. Jews found it harder to obtain government loans and licenses than non-Jews, among other forms of economic discrimination. The economic position of Polish Jewry deteriorated sharply

In 1926, a coup by Pilsudski of PPS led to a "soft" autocratic government, in which Pilsudski was in power but there was freedom of the press as well as parliamentary elections. Pilsudski did not believe in Jewish nationalism either, but this was because he believed that Jews should assimilate into the Polish majority, even if they stayed a religious minority. While he had no real love of Jews, he was, at least, anti-antisemitic. In the postwar era he had spoken out against the antisemitic pogroms that had plagued Poland, making him quite popular among Jews, and while his coup was the beginning of the end of true democracy in interwar Poland, the fact that Jews had a benevolent(ish) autocrat in power who allowed Jews on his party list felt good. However, Pilsudski essentially "solved" the Jewish problem by ignoring it, not focusing too much on the question of nationality in the first place. A major reason for his popularity, and the popularity of his government, among Jews was that even if he didn't necessarily specifically make things better (except insofar as he had taken the government out of the hands of the openly antisemitic Endek), he also wouldn't actively try to make things worse for the Jews. In a tumultuous and difficult time for Jews, this was seen as a win. When Pilsudski died in 1935, the Jewish community at large mourned him.

Pilsudski's death led to a great worsening of the conditions of Jews in Poland. Already beforehand, the Great Depression and the influence of Naziism (plus the signing of a nonaggression pact with Germany in 1934) led to a sharp rise in far-right political activity, including the formation of new political parties and the move to the right of existing ones (like Endek). With Pilsudski gone, antisemitic policy greatly increased. Jews were banned from the government's ruling party, which in 1937 established its policy on the "Jewish question" as attempting to defend Poland's culture and economic self-sufficiency by suppressing the Jews through economic means, though not violent ones (which was seen as uncouth). Emigration was also seen as a valuable tool to get rid of the Jews, which led to the government openly supporting Zionist movements, as well as other attempts to find a place for the Jews to go (such as Madagascar). Laws like the 1936 ban on ritual slaughter, the establishment of "ghetto benches" in (state) universities where Jews were segregated, and economic boycotts of Jewish businesses (often accompanied by physical violence) were crippling not just for Jewish material existence, but the Jewish psyche as well. Suicides sharply increased, a sense of lethargy and hopelessness prevailed (particularly among the young), and the Zionist movement gained renewed strength, ironically, as mentioned, bolstered by the very government which caused the Jewish community so much grief.

As the 1930s went on, Jews inside and outside of Poland quickly came to the conclusion that there was no future for Poland's Jews. They could not know exactly how true that statement would come to be, and how.