I believe that the difference is that some in the Republican party—McConnell, for example—explicitly advanced the position "whatever Obama does, we will oppose." Whereas I am aware of no mainstream Democratic politician who stated a blanket opposition to anything Trump might do; it just happened that they very often disagreed with him on the things he did do.
I think bad-faith accusations come from opposing a person's positions ad hominem, because of who holds them, rather than from simply disagreeing, even consistently, passionately, and vociferously, with many or all of the positions that a person happens to hold.