I'm going off the assumption he's losing Wisconsin, which seems pretty certain to me based on polling. I think a loss right now in this state fuels the perception that he's on his way down, even though that's a bit unfair. (For instance if the NY primary were this weekend, he'd be a huge winner)
Im also jumping the gun a bit, as I honestly have no idea what the convention will be like and whether it's really possible to strip him of 1100 delegates...
But I feel strongly that he's done because the establishment isn't in his corner. They must feel very strongly that they can take it from him.
When I first got interested in this race, I figured the establishment would fall in line, and if they had I firmly believe Trump would pose a big threat to Hilary.
But the RNC would obviously rather detonate their party than allow Trump to win, and so that's what they'll do. If they thought he was going to win at the convention anyway, Id imagine they'd be mending fences right now, and that's not what I'm seeing.