This year, Democrats only need five out of twelve seats to take back the Senate. Robert Reich points out that a great many Democrats running for the Senate this year are progressives.
Is this a Sanders effect, or a Trump effect?
In any event, it's important to vote down ballot. As it becomes clear that Hillary Clinton will be the next President, even she is adjusting her campaign's focus to ensure that there's a push for a Democratic Senate as well as holding on to the White House.
Needless to say, Trump is not helping down-ballot Republicans, and his candidacy is hurting them. 538:
The nightmare scenario for the GOP is that high-information Republican voters, seeing Trump imploding and not necessarily having been happy with him as their nominee in the first place, feel free to cast a protest vote at the top of the ticket. Meanwhile, lower-information Republican voters don’t turn out at all, given that Trump’s rigging rhetoric could suppress their vote and that Republicans don’t have the field operation to pull them back in. That’s how you could get a Clinton landslide like the one the ABC News poll describes, along with a Democratic Senate and possibly even — although it’s a reach — a Democratic House.