Responding to the Republican regime
The vast majority of what the new minority-rule regime tries to do will be bad, and must be resisted. That's beyond dispute. But it's possible that from time to time some element of that regime may propose something positive. The most likely "element" to do this is Trump himself, whose positions are notoriously changeable and non-ideological.
As an example, at various times during the campaign Trump suggested he'd support raising the federal minimum wage, though of course at other times he's said the opposite. If at some point during his term he actually proposed doing this, it would be absurd for Democrats to oppose it simply because he was the one making the suggestion. The same would apply if he eventually reacted to a prolonged debacle about "replacing Obamacare" by proposing a single-payer system, something he's also talked about in the past -- or if he made an unexpectedly moderate nomination for the Supreme Court. In the unlikely event that he actually did any of these things, he wouldn't get much Republican support, but there are past examples of Presidents working with the opposition party to pass things their own party objected to.
The average person struggling to survive on the minimum wage or worrying about health insurance doesn't give a rodent's posterior about whether some politician (or blogger) succeeds in remaining unsullied and pure from the taint of cooperation with a Republican. He just wants a better minimum wage or more secure insurance. If you're a member of Congress, your job is to serve the interests of that person, not to show off how strong of an absolutist posture you can adopt. Pragmatism dictates that when an opportunity arises to do some good for the country, that good must indeed be done. Real leaders get this, with Pelosi, Sanders, and Schumer having already said they would work with Trump if he ever does offer any progressive proposals.
The pragmatism of political gain points in the same direction. It will be argued that if Trump were to propose (say) a higher minimum wage, Democrats should not help pass it because Trump would get the credit. However, Democrats would get some of the credit, especially if most Republicans noisily resisted the idea. And if Democrats responded to such a scenario by blocking the increase, Trump would still get the credit for trying, while Democrats would get the blame, and millions of low-wage workers would not get the increase they need.
It's important that this be grasped not only by Congressional Democrats, but by activists and ordinary people as well. Mass public opposition can actually force Congress to change direction, as the recent attack and sudden reversal on the Office of Congressional Ethics showed. This is an encouraging sign for the next four years of dealing with the overwhelming majority of the Republican agenda which is unequivocally destructive and must be resisted. But we need to be prepared to respond properly to any opportunities which arise as well.