We've been seeing a re-alignment of the electoral map all primary season, but how far could Trump actually shift things in a single election?
Since he is the re-incarnation of McKinley, the closest parallel we have to study is the 1896 election. Judging from that example, I've tempered expectations about his Electoral College victory away from a 1980 style landslide, and more in the 300-340 range. When the country was just beginning to leave the bitterly divisive Civil War and Gilded Age period, it was unlikely that the President would win in a landslide that suddenly united most of the map.
Still, the electoral map was re-shaped, however slightly, in ways that would grow over time.
How did McKinley win?
First, he won two key swing states -- Indiana and New York, the latter having by far the largest number of Electoral College votes at the time. These two gave him 11% of the entire EC, which today would be around 60 votes. If Trump wins the swing states of Florida, Ohio, and North Carolina, that would give him as much as Indiana and New York gave to McKinley.
Second, McKinley actually lost a few red states to William Jennings Bryan -- most of the Plains and Mountain region. This was just a temporary loss, with most won back in 1900, and all back by 1904. So these losses were not part of a re-alignment. And they only defected to the Democrats in 1896 because they were running a populist in the vein of Bernie Sanders. However, with the Wall Street warmonger leading the Democrats this time, it's unlikely any of those rural states will get peeled away.
Third, McKinley won several traditionally blue states, such as Kentucky, which was a fluke and went right back to blue in the next election. This one state gave him 3% of the EC, which today would be about 15 votes. Trump could do this by temporarily winning New Hampshire, Maine, and Connecticut -- which might go back to being blue in the next election, when there is a Democrat more to their New Englander liking, instead of Crooked Hillary Clinton.
McKinley also won the reliably blue states of West Virginia, Maryland, Delaware, and New Jersey. These were lasting shifts from blue to red that would last throughout the Progressive Era led by the Republicans, and only returning to blue during the New Deal. Before 1896, Democrat meant anything south of the Mason-Dixon Line. When these states were shifted to red during the Progressive Era, Republican no longer meant "Yankee abolitionists" but "industrialized economies". So these four joined their fellow industrialized states of the Northeast and Great Lakes.
These four states gave him 6% of the EC, which would be a little over 30 votes today. Trump could repeat this by peeling off reliably blue Pennsylvania and Michigan. Just as in 1896, this would be the start of a re-alignment toward Republicans representing industry, manufacturing, and recovery of the Rust Belt.
If Trump doesn't lose any of the Romney states (likely), and adds the states described above, he'll wind up at 304 electoral votes. I think he's got a decent shot at Oregon and Nevada, which would boost the total to 317, right around my best guess of 320 (give or take 20).
The important lesson to remember is that in a re-alignment election, the winner doesn't only "win enough of the swing states" as in the more conventional election. Rather, he peels off several states that had reliably been won by the other party for awhile, and these changes are going to endure for several decades, as the parties re-define what they're all about and who they're appealing to.
Moreover, the re-drawing of the map will be mostly asymmetrical, favoring the winner. Trump is going to peel off far more blue states than Clinton will peel off from the red column (likely zero). During the Progressive Era, the Republicans peeled off those industrial states just below the Mason-Dixon Line, along with New Jersey and Connecticut. The Democrats did not peel off any red states for good in 1896, becoming further and further confined to the South and Texas.
During the period that we're now leaving behind -- the liberal vs. conservative culture war -- the re-alignment favored the Democrats, who in the 1990s peeled off the West Coast and New England from the reliably red column. Those gains assured they would coast through for the entire period. Bush Jr. only interrupted their reign by having his brother halt the Florida recount in 2000, and having Karl Rove pay for the Ohio election to be rigged electronically in 2004.
We can also see how the party purity mindset seals the doom of the loser in the re-alignment shake-up. It goes without saying that pushing the conservative side in the culture war doomed that side to failure by alienating the West Coast, which had been reliably Republican for over 100 years.
I'm sure there was something similar going on with the Democrats during the Progressive Era. They were probably smugly content to abandon West Virginia, Maryland, Delaware, and New Jersey because they weren't Southern enough, and were too corrupted by modern industry rather than traditional agriculture. Hey, if you want to keep losing, by all means keep raising the purity threshold for admission into your elect club.
In our New Progressive Era, we will likely see the Democrats return to being the tiny hardened core party that alienates former members of its big tent. Their base will be the Lutheran Triangle plus Illinois, and the ACELA corridor. Anyone who isn't a hardcore practitioner of Minnesota Nice, an ACELA striver transplant, or a FedGov parasite, will be too unclean to join the Democrats, who will implode into being the party of yuppies and schoolmarms -- self-designated elites and know-it-alls who can't stand the common people running their own affairs along populist lines.