Paul W. answered 04/09/19
Dedicated to Achieving Student Success in History, Government, Culture
The question is somewhat problematic because it is dependent upon one's definition of what constitutes a Middle Class. In a sense, the term is often defined in the negative, 'The Middle Class is composed of people in society who aren't members of the Lower and Upper Classes.' Not very helpful because this, in turn, depends on how one defines the Lower and Upper Classes!
With this in mind, I've always understood that membership in the Middle Class was generally associated with the practice of a particular profession, meaning an occupation that required a rigorous form of formal education. I'll be the first one to admit that this definition is far from perfect...
In any case, I think it is fair to say that the numbers of those members of society who can be characterized as belonging to the Middle Class did indeed increase during the course of the 19th and 20th centuries in, at least, Western and Westernized nations. It's been my understanding that this phenomenon was primarily due to the effects of the Industrial Revolution, producing an increase in a demand for formally educated employees to assist in the running of increasingly complex businesses.
This was part of a cyclical process because the rising number of members of the Middle Class, people who earned wages that gave them greater purchasing power than members of the Lower Class, produced a growing market for many of the products of the Industrial Revolution. This drove the establishment of a more widespread consumer culture, along with rising demand for a variety of forms of entertainment.
In addition, the growth in the number of members of the Middle Class created a growing need for institutions of formal education, first beyond the level of Grade Schools and, in turn, beyond the level of High Schools. The growing demand for access to education also reflects the growing involvement of the Middle Class in political activities. In the case of nations with some form of democratic government, the growing numbers of Middle Class voters gave this section of society direct influence on the composition of the government and, therefore, the policies implemented by the government. Even in nations with non-democratic forms of government, the calls for change coming from the Middle Class could influence the decisions taken by political leaders.
As a general rule of thumb, the Middle Class have served politically as moderating influence, a middle ground between those who call for violence to produce radical change and those who oppose any form of change at all. In this respect, As such, the Middle Class have played a decisive role in the increased liberalization, culturally, economically, politically, and socially, of Western Civilization during the 19th and 20th centuries.