Let's start with the last real 50-50 election.. when America was only beginning to become as polarized as it is today on regional lines. 2000, but with 2016 electoral votes.
Hillary Clinton (D) 254
An even shift of 4.48% of the popular vote based upon 2000 would give Hillary Clinton 55% of the popular vote. Anything more is superfluous, and anything less allows a 10% margin only if third-party candidates muck things up.
So you don't believe that map? Neither do I. For one thing, just about any Republican nominee is going to get more than 53% in Kansas, 51% in Alabama, and 55% in Oklahoma. That doesn't even show on the map. So let me do a little horse-trading of states. Hillary Clinton is much more likely to win Virginia than Tennessee and much more likely to win Colorado than Missouri. Make those two shifts and the map looks so:
OK -- that map looks all too familiar. Barack Obama won that map with 51-47. But Hillary Clinton is not as polarizing as Barack Obama, as shown in recent polls. I told you that I didn't think that Hillary was going to hold a Republican nominee to vote-counts under 55% in Alabama, Kansas, or Oklahoma. But to concede those votes 'back' to the Republican I must take them from somewhere. Hillary Clinton is not going to get 60% of the vote in Michigan, Minnesota, or Wisconsin either.
I've seen plenty of polls for North Carolina, and so far Hillary wins every one of the matchups. The potential nominee who wins Arizona loses Arkansas -- and vice-versa. Georgia looks close enough in early polling. Missouri and Indiana just don't get polled much. Your guess is as good as mine on the states in white.