In our system of government there are so many veto points that even an out-of-power party can do quite a bit to stop progress (mostly by way of the fillibuster in the Senate, and the president's literal veto power). This means that to have a functioning government one of two conditions needs to be met: all the branches need to be controlled by the same party, and in the Senate that must be a supermajority (60 senators), or the minority party in the Senate must consent to allow the majority party to govern, and/or (depending on who controls what) the two parties must cooperate and work together.
It makes sense, however, for the out-of-power party to do all it can to stop the in-power party from achieving anything perceived as good. Republicans, and only Republicans, have taken this to heart and understand that even if they control Congress, anything good (or bad) would get credited to the president (not really fair but that's the way it goes).
So it makes sense for the party that is not controlling the executive to make things in the country worse, so that the voters are more likely to change the control to them. Republicans are perfectly content to harm the country so that they can gain in future elections. If Democrats were willing to do the same, this might balance out but they are not. Democrats, unlike Republicans, actually try and accomplish as much good for the country/their district as possible whether or not they are in power. This means that they cooperate with Republicans to make bills more to their liking. It also means that if they succeed the Republicans get credit, which means a weaker position in future elections (so Republicans are more willing to cooperate with Democrats if Republicans control the presidency).
This has the natural result of politics that will favor Republican policies over time. This is true even if (as is actually the case) Democratic policies have more support in the electorate, and (as is also true) the evidence indicates they are better for [the economy, the planet, people's welfare, happiness...].
So why don't Democrats play the game the same way. There are logical reasons for this too. The main one is that Democratic voters are far more likely to believe that compromise is a good thing to do. This has gotten less true of late, and is less true when applied to politics than to life in general, but it is still the case.
Republicans play politics as a zero-sum game. Democrats do not. This means that, frustratingly for any thinking person: Republicans are more likely to win in the long run, and the US is more likely to lose (note: not because of policies, though I think that too, but because periods where Democrats are in charge, Republicans are fine with hurting the US to gain power back, but the reverse is not true).
The twisted logic is that it could be that voting for Republicans may be what is best for the country and best to advance Demcratic priorities, and that this will continue to be true so long as the two parties are playing different games. Either Democrats need to wise up and play the same game as Republicans, or voters need to get over racism, sexism and xenophobia to actually vote for the party that has their interests in mind. The past few decades hasn't left me with positive feelings about the Dem establishment, and after this election, I don't have much hope for the latter.