Newt Gingrich campaigning for Trump in 2016. Getty Images.In 2015, I made the biggest mistake of my career by being too dismissive of Donald Trump’s chance of winning the Republican nomination. But I had my reasons, even if they weren’t great ones. The “lesson” of 2012 had been that populist or insurgent candidates — Herman Cain, Rick Santorum, Newt Gingrich, you name it — could surge for a time in the polls, but wouldn’t survive sustained scrut…