He's not very popular. He has terrible approval ratings. Somewhere between 65 and 70 percent of Americans dislike him. It's depressing that it isn't higher, but still you would think that leaves him no chance.
The issue is that here in the US, we have trouble getting people to vote. The people who do like Trump are maniacs. And you can't dissuade them with logic, because if they used their brains they never would have embraced Trump in the first place.
His path to winning is basically to get his hardcore 30% to all turn up. Then maybe get another 5% from Republicans who do not like him but feel they somehow must make a stand for their party. Then hope that of the remaining people, not enough of them show up to vote.