Four million conservatives stayed home in 2012. Granted, you have some NeverTrumpers, RINOS and others who will vote for someone else or stay home, but that number won’t even come close to four million. Add a large number of independents and crossover Democrats and Trump will have a much larger total vote than Romney had, and in the battleground states that will be a huge difference.
Plus it seems that Trump and supporters are finding fraud and being proactive about it whereas Romney didn’t.