The lopsided distribution of wealth. Power follows money more than anything else. Through powerful influence over the media, elections, and markets, the top percents of wealth holders use that money power to insure that their interests become continuously more secure, and this is often at the expense of the middle class and poor.
They use the media to pacify you first, make you a passive bystander to a real life. Then if that doesn't work, they use it to distract you with a myriad of relatively inconsequential controversies. If that doesn't work, and you make too much noise, you will be marginalized.
In elections, they use money to buy politicians of both parties. When the electorate is especially duped, they vote for republicans. When they are getting more uppity, they vote for democrats. Neither party effects any real change to the fundamental interests of wealth holders.
In markets, they use their market power to pay as little as possible and charge as much as possible, both of these for as long as possible. The result is that, for the middle class, they will always struggle.
For a while, many of the people with power thought they should fear what the people would do if too much wealth accumulated in the hands of the few. They now know that they have us all sufficiently duped that I think they don't give it much thought. And so the system gets progressively worse, year by year.