2016 proved that tough primaries don't matter. Donald Trump went through the most divisive GOP primary in decades, several of his opponents outright refused to endorse him or explictly said he was unfit to serve and much of the party still hadn't united behind Trump by November, but he won regardless. Obama also had a rough and divisive primary against Hillary Clinton in 2008, while McCain glided to victory in February, yet Obama crushed McCain in a landslide.
Trump barrelled through his primary not changing his message one iota for the general. Romney tried to pivot from being a "severely conservative" advocate of "self-deportation" to a standard business Republican.
