The next Senate will consist of 51 Republican Senators, 48 Democratic Senators, and 1 Independent (Sanders). All Democrats have to do to block a vote is get 1 Republican disillusioned enough with Trump to vote no.
I'd say Jeff Flake and Ben Sasse would be good options. Both are fairly moderate (at least I think Sasse is, I KNOW Flake is) and they both really hate Trump.