Of course, one can't just assume trend will continue for no reason. (As a physicist, I recoil at the idea of political 'momentum.' There's something like it, but it's much more complex than that.) Trends can change rapidly.

That said, while a trend can change for the better, it's also possible for it to change for the worse. People tend to forget that side.

Like you (I gather), I'm concerned that a kind of 'scandal fatigue' is setting in, that events that have held Trump's ratings down thus far just start to settle into the background.

On the other hand, I'm also a little worried that Dems might do TOO well in the midterms, gain big power in the house, then overstep their mandate and make Trump more popular in 2020 (like the GOP did with the Clinton impeachment, though the timing was different).

I actually think the ideal outcome for removing Trump from office would be for Dems to come CLOSE to taking the house but not quite getting there in 2018. For that to play out we'd NEED the Dem lead in generics to shrink from where it is now. But I'm obviously overthinking it.