I'm not sure what the "duopoly" is that is referred to in the OP, so can someone explain that to me? It isn't referenced in the article that is cited in the OP.
Regarding that article, Fitch gave as its excuse for dropping the credit rating essentially two reasons:
1) The huge debt and record deficits
2) The toxic politics of periodic threats to shut down the government over the question of whether to pay its debts.
The first reason is what most of the discussion has been about in the thread, because most of the debaters seem to be fiscal conservatives who always believe that that is our main problem. There has been some Democrat-bashing, because conservatives think that excessive spending on social programs is wasteful, although conservatives have their own spending priorities--primarily tax cuts for the wealthy and corporate subsidies. So one can blame the spending on Democrats and Republicans alike. However, complaints over government spending have existed forever, and the US economy seems to be in good shape at the moment. We have relatively high employment, inflation has come down, and we don't seem headed for the long-predicted recession. So I think that excuse #1 by Fitch is completely bogus.
The second excuse strikes me as the actual reason. The toxic politics in Washington has led to
and it is likely to continue to do so. The problem is the law that requires Congress to continually raise the debt ceiling on debt that it has itself already accrued. The country keeps threatening to stiff its creditors. That is, it keeps threatening to become a deadbeat on the debt. So it does make sense, in light of those persistent threats, to lower the credit rating. And this action by Fitch came after yet another crisis brought on by the use of the debt ceiling to shut down the government.
If the US Congress were to repeal the debt ceiling limit and behave like the vast majority of other nations in the world, then its credit rating would go right back up again. Because the US economy, even with its huge debt, is still the largest economy in the world, and it still has one of the healthiest economies--except for the stupid debt ceiling law.