Terrorism/immigration being the main issues.
Easy to forget now, but Trump truly did look like his campaign was slowing down significantly in October/early November 2015. He hit his ceiling of 30% and that was going lower and lower. Around this time, Ben Carson passed him in multiple polls including an NBC one where Carson was at 29 and Trump was at 23.
Trump almost lost his first place lectern at the Fox Business debate. He was barely hanging on and seemed stuck at 25-30%. The "fad" appeared to finally begin it's end in early November.
Then something happened. Paris was attacked on November 13, 2015.
150+ dead (what people thought at the time). Hundreds injured. ISIS claims responsibility. Refugee IDs were found by some of the suicide killers.
All of a sudden Muslim terrorism is thrown at the forefront of the race.
When the attacks happened, I remember all over Twitter people were saying "This is why Trump will win" or "Trump just won the election today." Trump's campaign, which was slipping fast, all of a sudden made a comeback.
This is when Trump was saying stuff like "I saw thousands of Muslims cheer on 9-11 in New Jersey" or when rumors of a registration for Muslims begin.
Now his poll numbers start bouncing up to August levels, and his campaign is rejuvenated. Carson plummets as it's clear he has no idea about foreign policy.
Trump is now once again a clear number 1 in late November, but his ceiling still seemed to be around 30-35%.
Then the San Bernardino attack happened.
This is when Trump's numbers went REALLY high (really wanted to avoid a pun here!).
After his Muslim ban announcement, he was hitting new highs. A Monmouth poll showed him at 40+ for the first time. On betting markets he was now at 60%+ chance to win the nomination (before he was like 3rd place behind Rubio and Bush).
He would be a clear number 1 from then on. His loss in Iowa stunted his campaign a bit as well. But winner take most/all states like South Carolina, Florida, Missouri, and Indiana would make him the presumptive nominee.
One thing I am thankful for due to the disastrous Bush administration and Iraq war is that neo conservatism has been completely and utterly disgraced. Presidents since then and into the future for at least the medium term will be extremely gun shy and afraid of getting into another nasty quagmire. We are safe at least for another 10-15 years from another Iraq style war. The US had 15 years between Vietnam and the 1st Gulf War.
