One of the greatest sources of confusion about how the Electoral map may change in any given election is the misleading idea of "swing states". It makes it sound like they flip flop, and with the race so close otherwise, it all comes down to these few states to swing the election one way or the other.
In reality, the only states that show flip flopping are Ohio and Florida, and the only times that they've swung elections were due to shenanigans that gave one of them to the wrong party. So it's more accurate to say that, flip flopping or no flip flopping, a heavy-handed case of shenanigans can swing an otherwise close election.
The secondary usage of "swing state" is to refer to "close" states, where the margin of victory is under 5 or 10 points. But these are all reliably blue or red (mostly blue in our period). Here, "swing" is being used delusionally to suggest that if only we tried really, really hard, we could swing it from one color to another. But if all the blood has been squeezed out of the stone, that's it. It doesn't matter if the margin was under 5 points -- it ain't gonna budge any further.
Getting back to the primary usage of "up in the air," let's start with the states other than Ohio and Florida and explain why they're not swing states. We need to restrict our time period to one where most of the map was predictable, and so where only a handful of states could have changed from one year to the next. That means the culture wars period, from 1992 onward.
Nevada, Colorado, and Virginia began red and have steadily shifted blue over time. Although this does mean that some years are blue and others are red, it is not flip flopping, which suggests that any given year is up for grabs. They're only mixed colors because they started out one color and have steadily changed toward the other, a deterministic process.
Nevada and Colorado did narrowly go blue in '92 and '96, but only because of a high Perot vote, which split off more Republicans than Democrats. Their underlying nature, in the absence of a strong third party, was still red. Many other states went blue only due to Perot, despite being red states, such as Montana and Kentucky, which does not make them "swing" states.
The cause of this temporal shift toward blue is the migration of liberal transplants into the Las Vegas, Denver, and Northern Virginia (DC) metro areas. Unless and until this trend reverses itself back to the level of the mid-2000s, these states will remain blue.
Iowa has only gone red 1 out of 6 times, not flip flopped. New Hampshire went red once, too, but even that may have been due to Nader splitting off Democrats in 2000. It was still an underlying blue state. New Mexico went red once in '04, perhaps because Bush promised to keep their housing bubble inflating.
Indiana and North Carolina went blue in '08 as a one-time referendum against the neo-cons. North Carolina is not quite as red as it used to be, subject to the same liberal carpet-bagger process as Virginia, Colorado, and Nevada -- bringing them into Research Triangle.
A handful of Southern states went blue in '92 and '96 because of Clinton and Gore hailing from Arkansas and Tennessee, and being a supposed throwback to the Southern populism of an older Democrat party. Louisiana and Missouri joined these two. There was nothing "up for grabs" about them, though, and they have been solid red since 2000.
West Virginia started out blue in '92 and '96, but has steadily shifted red since, so nothing up-in-the-air about that one either. They weren't glomming onto Clinton and Gore as local heroes, since West Virginia is not Southern, and it had already voted blue when the entire rest of the country voted red earlier on -- for Dukakis in '88 and Carter in '80 (also in '76).
Aside from one-off flukes, or deterministic processes (long-term shifts in one direction or the other, local favorites, etc.), only Ohio and Florida show something like flip-flopping from one year to the next.
Ohio has officially gone blue 4 times out of 6, although in '04 the election was electronically stolen away from blue, making it blue 5 times. But in '92, it only went blue because of the size of the Perot vote, so that's still an underlying red state in 2 and blue in 4. And it wasn't a steady shift from one color to the other. In '04, its (rigged) outcome determined the entire election. So it's safe to call Ohio a swing state.
Florida has officially gone blue 3 times out of 6, although in '00 the recount would have shown it to have gone blue, so underlying blue for 4 out of 6. It's changes are not steady shifts in one direction -- it was red in '92 and '04, and was illegitimately red in '00. And its shenanigan-driven outcome in '00 determined the fate of the entire election. So it, too, is safe to call a swing state.
Notice, though, that the only times these swing states have swung an election was with the help of some kind of blocking of the popular vote. So it's electoral shenanigans that have swung elections, not a changing popular mood in up-for-grabs states.
Time periods tend to have a dominant party that represents the zeitgeist, and clearly it has been the Democrats during the culture wars period, given that more Americans are liberal than conservative. If only the popular vote mattered, they would have been in office the whole period, from Clinton to Gore to Obama.
In 2016, none of the deterministic processes has reversed (liberal transplants leaving former red states to make them red again), so not even the swing states can truly swing an election. If McCain and Romney had won Ohio and Florida, they still would've gotten whipped.
What will turn the White House over to a Republican again is a re-alignment of which kinds of people and which states vote for the newly evolving Republican party under Trump. Likely this will be through the Rust Belt. If Trump wins, it will probably be by winning at least one of the swing states and some but not all of the Rust Belt. Over time, more and more of the Rust Belt will turn red for the Trump-oriented GOP.