Explain how the populist party influenced 20th century politics, even after it's decline

To me, the major influence of the People's Party (the Populists) is that it did a great deal to lead to the rise of the Progressives in the early 1900s. As an often-quoted line goes, populismshaved its whiskers, washed its shirt, put on a derby, and moved up into the middle classWhen it did this, it became Progressivism. The Progressive agenda was really based on the Populists' agenda, only with much less of an emphasis on farmers and rural people.In the longer term, you could argue that the Populists' ideas have been very enduring. They have (you could argue) led to modern day politicians like Sarah Palin who believe in the idea of "common" or "regular" or "real" Americans fighting a battle against the elites.

why was the national assembly formed by the third estate?

the estates-general had not been assembled since 1614, and its deputies drew up long lists of grievances and called for sweeping political and social reforms. the third estate, which had the most representatives, declared itself the national assembly and took an oath to force a new constitution on the king.