The Birther Vote.
in 2010, 44% of repubs didn't believe Obama was born in the USA.
in 2012, Trump made himself their voice.
In 2016, Trump won with.... guess what.... 40% of the votes (and it'll be 43 or 44% after the last few states).
Trump won because he's the birther in the race. He'll end up with about 44% of repubs' votes, and not coincidentally, about 44% of repubs think obama was born in Kenya. He branded himself for 6 years as THE birther leader, and he won the nomination because of it.