Dartmouth political scientist Brendan Nyhan put together a chart that dramatizes the extent to which a Donald Trump nomination looks like an unforced error for the GOP.
Hillary Clinton is intensely disliked by a larger share of Americans than anyone who's ever received a major party presidential nomination in the past. But Trump is even more widely disliked, and since elections are zero sum, this means a huge advantage for a candidate who seems kind of weak by historical standards.
If you pay attention to the historical data, you'll see that early disapproval isn't necessarily all that predictive — John McCain and Mitt Romney were both less disliked than Obama, and Mike Dukakis had very few haters back in 1988 (perhaps because he was relatively obscure). But the really big problem for Trump here is that he's over 50 percent, which would seem to make winning totally inconceivable unless he's able to massively shift public perceptions.