Is all about the economy.
If Trump succeeds in reducing regulation and taxes while pumping up infrastructure spending. And the economy has robust 4% growth resulting in real income growth for the middle class then Trump can win a a 5% popular vote victory and 355 EV.
If Trump's economic plan blows up the deficit, ramps up inflation, worsens inequality and leads to a recession he will get crushed.
Thing is, the economy tanking is more likely than most people realise. Remember in 2005/6 when everyone was convinced that the economy's was doing wonderfully and there was no risk at all?
Two years later, everything went bust.
If it does tank again, then Trump will be lucky to break 20%. It gets worse for him in that situation if the Democrats nominate someone like Elizabeth Warren, who's known for tough stances on Wall Street.