In 1964, the leader of the extreme right wing of the Republican party, Senator Barry Goldwater, won the presidential nomination at that year's Republican convention. He (and his party) lost in a monumental landslide. But, four years later, the country elected a Republican president anyway (albeit a moderate centrist, Richard Nixon). By 1980 the right wing had both regained control of the party and put one of its own in the White House. Soon the moderate centrists were gone, and the Republican party was a much more ideologically right-wing institution.
The Democrats went through something similar over the same period. In 1972, the leader of the party's left wing, Senator George McGovern, won the presidential nomination andthen lost in a monumental landslide. The next two Democratic presidents (Carter and Clinton) represented the more moderate centrist wings of the party. But the trend was inexorable, and soon most of the moderate centrists were also gone, and the Democratic party has become a much more ideologically coherent left-wing institution than it had ever been before.
Conventions don't matter as much (in terms of securing the nomination) as they did in 1964 or 1972. But they still serve as symbolic turning points in a party's identity. For decades, the Republican party's ideology has been about increasing the wealth of the already rich, top sliver of the population. But, to appeal to a sufficiently broader constituency to win elections, it has moved in a rightward direction on a host of social-cultural issues. This began with Goldwater's vote against the 1964 Civil Rights Act, positioning the (Republican) party of Lincoln to trade places with the (Democratic) party of the Old Confederacy as the vehicle for white racial politics. That appeal became more explicit with Nixon's "Southern Strategy" (more an electoral than governing strategy, since Nixon for the most part governed moderately). As the country has become more polarized, the process has accelerated. The Republican elite has doubled down on its tax-cuts-for-the-rich ideology, while continuing to address non-rich voters with social and cultural - racist, xenophobic, misogynistic, and homophobic - appeals. The elite tail kept wagging the populist dog until Donald Trump unexpectedly administered a monumental defeat to the establishment in this year's primary process, a turning point in the party's history which will be ratified formally at the Cleveland Convention this week. An establishment which had long winked at its base's de-legitimizing of the country's first non-white president has now been largely de-legitimized itself.
Political predictions are a risky enterprise. Human factors will always interfere with political science models. Still it seems reasonable to suppose that, if Trump wins the presidency, the Republican party will no longer predominantly present its elite, neo-liberal, economic conservative face to the world and will become more of a white nationalist party, similar in some respects to some contemporary European versions. If Trump loses the election, there will be more of an opportunity for the establishment to fight to regain control of the party from its more populist base. But, if the lessons of 1964 and 1972 hold, the transformation wrought this year by the party's grass roots may prove inexorable and irrevocable.
Meanwhile, the Democratic party has also been going through its own internal identity conflicts, all of which would have been much bigger news this year had it not been for the Trump phenomenon among the Republicans. The Democrats are nowhere near so internally divided as the Republicans. They also lack the same extreme allergy to the responsibilities of governance and so have more of a positive program to offer their base. And they have the country's changing demographics largely on their side. So their internal stresses and prospects for fractures are much less explosive. Still, when the dust has settled from this election, the Democrats will likely also continue to struggle among themselves over the soul of their party with by no means easily predictable results.