The same way the USSR lost. People talk about final-point reasons like economic issues caused by an arms race or whatever, but that's just the final steps of a long lost race. Empires collapse long before they finally fall due to inertia alone. And I think the "real" cause is always the same. When enough people inside an empire want it to fail, that empire will fail.

And a common cause for creating that sentiment (that applies to everything from Rome to the USSR to the US) is Empires naturally tend to spend an increasingly disproportionate amount of their time focused on affairs outside their borders rather than within them. You have urban left leaning types becoming increasingly anti-capitalist, and the more right leaning and rural types feeling neglected by both internal and external policy.

People agree on less than ever, but the one thing they all agree on is that the system sucks. This will likely result in over more radical shifts between presidencies. You end up with a country that's starting to feel a lot like a boat being rocked back and forth with increasing vigor. And obviously this isn't just the US. It seems many political systems throughout the world are headed towards dramatic shifts.