Trump didn't win by a slim margin at all, he lost the popular vote, but only largely because of California. He had early calls in several states that Hillary was supposed to win. Hillary lost states she never stepped foot in, she was just not a strong candidate, she didn't campaign, she had little ground game and conceded states simply because they were a "firewall." Trump should have never been within spitting distance of any Democratic candidate, especially after Obama who was fairly popular among moderates.