Despite challenging a sitting president in '76, Reagan didn't catch fire in '79 until the end of the primary.
Trump is much different from Reagan. Reagan was a staunch conservative with a track record and coherent philosophy.
Trump's track record is mixed, and it's hard to discern a philosophy. His positions seem to be ad hoc, which isn't surprising considering his background as a businessman.
The jury's out, as far as I'm concerned, and I'm disappointed that he's more popular than other great candidates, like Cruz, Walker, Jindal, Carson and more.
But there has to be something to the Trump phenomena that I'm missing. It could have to do with his boldness, courage in dealing with the media, and his plain speaking. OTOH, it could simply be that people already know him and are comfortable with his persona.