To start off with, we must recognize the underlying reason for all democratization in the United States. This is the fact that the US was created as a democratic country. We started with the idea that governments had to exist by the consent of the governed. This puts a great deal of pressure on us to ensure that all of the governed are allowed to participate in things like voting. We have not always lived up to this ideal, but it is always there and it always pushes us toward greater democratization.
There were, however, other things that moved us towards greater democratization during this time period. Let us look at three general causes.
- The rise of the middle class and progressive values. In the late 1800s, the booming American economy created a large middle class. This middle class came to hold progressive values such as the belief that the business elites should not be allowed to act as they pleased. This helped lead to the democratization of the progressive era. Women were given the vote to help the middle class curb the power of the elites. People were allowed to vote directly for senators for the same reason.
- World War II and the Cold War. These conflicts were centered around the idea that democracy is the best form of government and were fought against systems that were not democratic. In the case of Nazi Germany, we fought a system that was blatantly racist as well. These things put pressure on us to extend true democratic rights to African Americans in the 1960s.
- The Vietnam War. In this war, many people between the ages of 18 and 20 were fighting and risking their lives. It made no sense to allow (or force) them to do this if we were not going to let them vote.
Overall, our country’s values push us towards allowing greater democracy. In the time period that you mention, various specific events helped cause us to give more democracy to various groups.