As long as Trump’s approval stays above ~32%, they keep the Senate–with no real risk of losing it until 2022.
A few more serious hits and I may change my fairly dismal 2018 predictions.
That said, further progress would involve changing the minds of people who approve of Trump today, so it would take quite a lot. Each percentage point is harder-fought than the last, especially when you’re digging into core support.
Also worth noting–once Trump hits or slips under ~32%, Republican Senators up for re-election may show a sudden interest in joining their Democratic peers in the congressional oversight process, as a way to improve their odds (i.e. erasing their history of stonewalling for Trump). Which would also be a good outcome, one that could yield results before 2019.