They diabolically oppose Trump’s actions? Probably, but I think the author meant “diametrically”
Diabolic: Related to or coming from the devil (diabolos).
Diametric: Relating to the furthest points on a circle. More precisely, relating to the end points of a line segment which passes through the center of a circle and which extends across the entirety of a circle. By metaphor, two opinions, viewpoints or beliefs which are as far apart as possible from each other.
Of course the writer intended “diametrically,” but he instead committed a perfect Freudian slip. I’d like to think he caught it but, upon reflection, decided to let it stand. ;^)
Diabolic: Related to or coming from the devil (diabolos).
In this case,either one probably works just fine.