Before Trump, I would probably have agreed with you, but these days, I don't know how you can say that. Trump has made major breaks with the foreign policy establishment over the past 3 years, and continues to do so (pulling half of our troops out of Germany is a good example of this). On labor rights, the two parties have been far apart for years. The GOP has intentionally and willfully attacked and degraded the National Labor Relations Board, allowing it to lose quorum and be sidelined.
If you broaden the policy proposals to health care, the parties are even further apart, with one party advocating for single-payer state funded health care, and the other advocating for the immediate elimination of subsidized health care for over 10 million people.
There may have be some issues where there is a general consensus between the parties, but there are plenty of very important issues where the parties are far, far apart.
At best, the establishment Dems have been passive observers to the catastrophic erosion of social systems over the past 50 years.
> Both parties broadly promote global US military and economic supremacy (unipolarity)
They seem to be doing a pretty bad job of preventing the rise of China as a second pole.
It doesn't mean that advocating for multipolarity isn't a political dead end in the US. Both parties are against it.
Maybe that's the goal but the execution is lacking. Seen from outside the USA it seems that the influence and the prestige of the United States are lower now than at any time in my life. The apex was any year in the 90s.
Obama started the trend. Trump accelerated it. Dealing with allies as if they were enemies doesn't help to keep a supremacy. Everybody needs friends, even big countries.