The US's antiquated and ossified political system with its focus on minoritarianism and gridlock-as-a-design-objective are a major cause. As long as it's possible for ~30% of the country to drag everything to a halt, not a lot is going to get done. The push to restrict voting rights only amplifies that, but the real problem is that the US system is not built to produce a "mandate to lead", it is designed with an excessive focus on "checks and balances" that require supermajority control of all branches of government to accomplish the most basic of functions. Checks and balances and minority representation are good concepts in general but taken to too much of an extreme you end up with a government that cannot react to obviously changing circumstances, which is where we are now.
That's the thing that's functionally going to put the US into decline, our government is simply so gridlocked that it cannot accomplish even its most basic functions, and it's a result of how the government is structured that allows and encourages this. If China and Russia don't do it alone, climate change and other problems certainly will.
It's analogous to the liberum veto in the Polism sejm - the system "promoted bipartisanship" by allowing the current legislation under debate and all previous legislation passed in that session to be vetoed at any time by a single member. That's good, right? You have to pass things that only everyone agrees with! But it was exploited by bad actors both internally and foreign powers who wanted to weaken and disrupt the government, to the point the polish state collapsed into a state of anarchy and provincial rule. Sound familiar?
Ultimately it is better to have a system of government that produces an administration that is capable of accomplishing its policy objectives, even if that means that your political opponents can also do things that you don't like. You can then structure the rest of the government around making sure that the administration remains representative of the population. But having a government that is routinely unable to do anything at all is a design flaw with the system, not something to be cheered, and that's the primary reason the US is in decline. It's better for somebody to be at the helm, than everybody arguing over which way to go and nobody driving.
On top of that the US tends to conceptualize "unlimited rights" to a much greater degree than virtually any country. That makes it far more susceptible to misinformation campaigns. The internet is the Atom Bomb of the information age - it makes misinformation far more deadly than it ever was before, because now you can take misinformation straight to the idiots themselves on a micro-targeted level. Certainly misinformation was a problem before, particularly with radio (eg Father Coughlin) but that was eventually curbed with broadcast regulations. As those regulations have eroded the problem got worse again, and now that we have private infrastructure that is totally unregulated it is becoming vastly worse again. Even worse than before in fact, because now you can micro-target the lies each person is most susceptible too. It's just a completely different ballgame than it was 30 years ago, and the regulatory scheme hasn't adapted to the idea that pretty much anybody can wield this potent weapon. The whole western world with its liberal (in the rights-based sense) philosophies are struggling with this but the US takes the most extremist positions in this area and is impacted the worst.
The lack of regulatory response to the rise of weaponized, micro-targeted misinformation campaigns is certainly amplified by the gridlock caused by minoritarian control at the government level, of course, these are all inter-related problems.