Thank gawd independent % gets bigger every day.
Except there's no evidence for that.
Okay, let's look at percentages since 1996
Democrat - 49, 49, 49, 55, 53, 48
Republican - 43, 49, 51, 45, 47, 46.
Third Party - 8, 2, 0, 0, 2, 6
A lot of people might call themselves 'independent", but the fact is, since the last major third party movement petered out in 1996, the GOP gets between 45-51% and the democrats get between 48-55%. So the window of "persuadable" voters is really only about 10% if you are being really , really generous.
Now, it wasn't always this way. There was a time when a major party shit the bed and nominated a nut, enough people voted for the alternative, even if they thought he was a bit shifty.
Goldwater in 1964 and McGovern in 1972. Both well outside the mainstream, for the time, and the voters rejected them.
But in this case, the very same people who voted for McCain in 2008, despite everything Bush did, were the same 45% who voted for Trump, despite Trump being a womanizing, mentally unstable scumbag.