"Around half the country" is an exaggeration. Even with a two-party system, in modern history no candidate has achieved the outright support of anything approaching half of the US population.

Consider that only a portion of the population is eligible and registered to vote, and even among them voter turnout is relatively low in this country. And then there's always a segment of undecided voters who just make a choice on election day, without supporting either candidate prior to that.

In this specific case, add to the math that Trump is doing poorly in recent polls, and is disliked by an unprecedented percentage of his own party.

Math aside, given the sheer number of disturbing, extreme things Trump has advocated over the past couple years... if Thiel supports Trump enough to give an endorsement at the GOP convention, this certainly seems to condone his behavior, which in my mind is a pretty extreme position.

I would argue against your phrasing. I think around half the country is voting against Hillary.