I never really liked the term middle class since there's no clear definition for it. The only useful class distinction is around class interests in my opinion. There is the capital owning class and the working class. People who own significant amounts of capital generate wealth from the capital they own while workers sell their labour as their means of income.
These two classes have directly opposing interests since capital owners act as the employers for the workers. People who own businesses want to reduce their costs and maximize their profits. This means paying lower wages and providing less benefits. Meanwhile, the workers want to maximize the pay and benefits they get for selling their labour to capitalists. This is the primary contradiction in the capitalist society.