In addition to everything else, Trump's nomination is suppressing the Republican turnout
That means that a very large percentage of potential Trump voters- many of whom are not accustomed to voting in the first place- won't make it to the polls on Election Day. And having a voter's support does a candidate no good if that voter doesn't vote!
Even worse for Trump, college educated male voters- a traditional Republican strength- are deserting him in droves. In their self-destructive foolishness, the Trumpsters ridicule and insult Republican voters who are dissatisfied with Trump rather than trying at all costs to woo them back. And one of the consequences is that even more of them than otherwise might do so are going to vote for McMullin or Johnson or even Clinton.
But while nobody in the Trump campaign seems to realize it, we aren't his biggest problem. His biggest problem is the college educated Republicans and Republican-leaning independents who are going to react to Trump by not voting at all. Nate Silver reports that early voting figures show that while Democrats are voting in roughly the same numbers they usually do, early voting in states whose Republican voters are largely educated is lagging badly. That's where a "ground game" would be essential to any campaign that had the slightest idea what it was doing. That's where attempts to woo back lost voters might at least to some extent come to Trump's rescue.
But it's not happening. Part of the reason is Trump's arrogant refusal to see the necessity of doing these things. Part of it is that in counting on his voters to show up at the polls because they're so highly motivated he fails to take into account the fact that many of them simply aren't in the habit of voting and despite their intentions simply won't make it to the polling place.
But even more, the Trump Effect that has doomed the Republicans at the presidential level (and, it seems, at the level of the Senate as well) isn't simply driving actual voters into the arms of the Democrats or third parties and independents. It's also suppressing the Republican vote.
Large numbers of normally Republican voters are reacting to Trump's nomination by not voting at all. That's disastrous news for any campaign. But it's absolutely fatal to one whose ineptitude combines with such arrogance that it's not going to even try to do anything about it.