I wouldn’t call this an internal poll. But Abdul Hakim-Shabazz, the pundit who commissioned it, is generally (though not consistently) conservative. So it might be fair to call it a partisan poll. And I don’t gather that Mason Strategies has much of an established reputation.
I’m in Indiana and I think this race is still probably Donnelly’s to lose. We like our incumbents. That said, he’s an incumbent who isn’t routinely hitting 50%.
I hope it is indeed Donnelly's to lose. I am very unsure about this race. If he does hold on, I would say that it's a sign of the Democrats maintaining their numbers in the Senate at a minimum. And that's about as good as it will probably get.