The share of corporate profits in GDP has been on a rising path for the past 40 years, while the wage share has been declining until recently.
At the same time, income inequality has increased sharply and intergenerational mobility has declined. While these trends are global in nature, the US has undoubtedly been at the forefront.
Look carefully at the changes in the last five decades, and consider the effect on politics. The 1% become more influential because they have more money to burn, while the bottom 50% just scramble for a living and have less time and money for politics.
The 1% are very influential in politics because they can buy politicians, think tanks, and news organizations. The professional classes of the next top 5% largely exist to service the top 1%, and mainly echo their opinions (or they lose their jobs).
The modern left is now mostly an alliance between the top 1% — who supply the money and the policy direction — and the welfare dependent — who supply the votes. Between them, they are destroying the middle class.
The Democrats became the party that was bought and paid for by the top 1%. It has become the mouthpiece for the top 1%.