I think Tacitus' most useful lesson for american politics right now would be the importance of political compromise.
When political polarization reach the kind of levels it's at in the US right now, politicians tend to sacrifice the well-being of the country to hurt their political opponents - which is bad for everyone.
I'm not sure how long you've been following American politics, but for a few decades now the Democrats have been compromising and capitulating at almost every turn while the Republicans have staunchly refused to do this.
The CARES Act was a compromise, the refusal by the House to enforce subpoenas was a compromise. Even their choice of presidential candidate is a compromise of sorts, see Biden's "nothing would fundamentally change" comment to donors. Where do you see obstructionism on their part?
A number of places, but just for example: Sanctuary cities. Even though they lost the battle on the federal level, the cities try their best to obstruct the enforcement of federal law.
In the political context obstructionism refers to attempts to delay or otherwise interfere with the legislative process. Which the Democrats may engage in, but I haven't seen any evidence demonstrating that.
Sanctuary cities do not obstruct the legislative process, they are just a form civil disobedience. This probably can be deemed obstruction of justice, but that has nothing to do with obstructionism.
[2] An old trick for maintaining the peace in a herd is to put out more piles of hay than there are horses, so the dominant horses can politic to their hearts' content but everyone still gets fed.
A big problem in the US is that Congress has abandoned so much power as an institution.