If Trump isn't doing poorly then he's not likely to kill Republicans down ballot. I postulated that Trump wouldn't matter that much because Republican voters could vote for Clinton or Johnson for President and Republicans down ballot. Trump's improvement might help Republican senate candidates somewhat but the chart below says that there isn't that close a relationship between the Presidential and Senate candidates.
That isn't necessarily bad news for Democrats. Trump has gotten closer in Colorado and Wisconsin, but the Republicans in those states are running far behind Trump. Despite Trump having healthy leads in Indiana and Missouri, Republican senate candidates are way behind him in those states. On the other hand, Trump's struggles in Arizona, Georgia, Ohio, and Iowa haven't brought a Republican lead below 13.7% in any of those states. It appears Republican and Democratic chances aren't that closely tied to Trump.
One thing Democrats bring up is that Hillary Clinton is going to have a vastly superior GOTV operation. She'll be better at turning out Democrats than Trump will be Republicans an that's going to help Democrats down ballot. That may be the case, but Republicans down ballot will have to do their own GOTV effort, something they've done well in non-Presidential years of 2010 and 2014.