In regard to Congress, 2018 was not much of a referendum on Trump. The Ds flipped the House, but the number of seats flipped in Trump's first midterm was anemic compared to Clinton's, GWB's, and Obama's first midterms. OTOH, the Rs have improved their majority in the Senate, effectively taking the Thomas-Kavanaugh tactic off the table in judicial confirmation proceedings and decreasing the power of R waffles.
The Ds gained the ability to obstruct and attack Trump in the House. But to the degree they do so, Trump gains a big fat target, to the same degree.
In the Senate, the once extremely slim chance of Trump being removed from office if impeached is now approximately zero. The Ds' ability to obstruct Trump's judicial nominees is much diminished (RBG hurt worst). Loony-tooniness the Ds' might originate in the House - if they go that route - will be blocked in the Senate.
In sum, I think Trump "won" a split decision, and the Ds' victory in flipping the House could be Pyrrhic if they let TDS reign.