New Republic's Jeet Heer points out how the Republican Party's "Sourthern Strategy," going all the way back to the 1950s, led more or less directly to Donald Trump's campaign:
Far from being a “cancer” on Republicanism, or some jihadi-style radicalizer, he’s the natural evolutionary product of Republican platforms and strategies that stretch back to the very origins of modern conservatism in the 1950s and 1960s.
The racist voters swarming around Trump didn’t just pop out of nowhere. The Republicans have been courting them for decades now, in a dramatic break from the party’s origins. From its creation in 1854 in opposition to the expansion of slavery until the 1940s, the Republicans were the party of the North, and more anti-racist (albeit sometimes only marginally so) than the Democrats, whose most reliable base of support was the “solid” white South.
The Southern Strategy was the original sin that made Donald Trump possible. If Republican voters were anywhere near as diverse as the Democrats’, a candidate like Trump would have been marginalized quickly. Conservative elites can denounce Trump all they want as a “cancer” or an impostor. In truth, he is their true heir, the beneficiary of the policies the party has pursued for more than half a century.
It's a long-ish article, worth the time.