What are the political and social changes of the United States with regards to populism

Since your question does not specify for which era, I will use the contemporary era for my answer.

Populism came back to the forefront of American politics during the 90s when Ross Perot ran for the US Presidency on a populist platform rejecting both parties and the Elite Establishment. It was a time of economic prosperity for some (tech workers, financial workers) and hardship for others (unskilled industrial workers) who lost their jobs due to globalization, when factories closed and moved to low-wages countries abroad.

During the Great Recession, Populism developed on both sides of the political spectrum. The left had the Occupy Movement and the right had the Tea Party movement and both shared a rejection of establishment elites. In the last presidential election, two Populist candidates were extremely popular, Donald Trump and Bernie Sanders. It can be argued that Trump, as a billionaire is not really a populist candidate but he did run on a Populist campaign, promising to "drain the swamp" (to remove corruption in Washington).

Since the 1980s, politicians have become increasingly technocratic, dismissing the views and opinions of the American people and deciding for them in very fundamental issues without even consulting them to confirm that their policies are supported by the majority. With the continuous economic and political crisis people of the left and the right are fed up with being ignored by politicians and want substantial participation in the decisional process of the federal and state government.