What has Trump done for his numbers to collapse nationally?

[Absentee Voting Ghost of Ruin]

Perhaps, as Trump's nomination becomes more and more likely, those polled are taking the question more seriously?

Before, some people who may have said "sure, I'd vote for Trump in this hypothetical matchup" are now being faced with the stark reality that it is likely actually going to be Clinton vs. Trump, realizing it's not fun and games anymore, and voting for the adult in the room.

This could even have a positive feedback effect, if the dyed-in-the-wool Hillary haters start abandoning Trump because "he can't beat Hillary".

[Why]

The more people who were wavering about Trump get to know him the less likely they are to ignore their misgivings and vote for him because he is the Republican. Everything he does that fires up his supporters adds more to the numbers of Republicans who are deciding they will not vote from him in the GE. Yes, some of them will come around when the consider the alternative more starkly in the GE climate, but some will not and that will hurt Trumps chances, probably fatally.

[Amenhotep Bakari-Sellers]

A 3-5 point victory is a landslide, 52/48 instead of 50/49 will give Dems a solid majority.  It didnt take much in 2012 for that to happen and Trump is a weaker nominee, Dems arent there, but as economy improves, they can get there.