What do you mean, "the remains of his base"? I sure haven't seen his base leaving. His base is now pretty much what constitutes the party. He routinely polls in 90% range re Republican support. And the vast majority of these people will never vote for a Democrat, I suspect even if they found a Democrat like Trump.
I just mean whatever is left after the "well, let's give it a chance" independents moved on (if they even existed in the first place). The firm base he's had for the past 2 years.
That's the question then. I agree they'll never vote Dem, but would they stay home? If not, the senate may be comfortable removing Trump since that base would likely translate to Pence (or someone else if they tried to put up another candidate).