Empirically, measuring diffuse support has always been somewhat challenging because asking people what they think about the political system will always tap into their attitudes towards those in power at the time. Still, the fact that we don't have revolutions is a demonstration of the fact that we do have high levels of diffuse support, compared to other countries.
I'll pose several observations today, then.
First, while the strength of support for any one party can wax and wane, as the title of the post suggests, diffuse support is very hard to build. The Republican Party had, let's just say, a hard time of things after Watergate. Then, 1980 came along. While Ronald Reagan the legend is rather different from Ronald Reagan the actual human being, the fact that Republicans took control of the Senate for the first time in decades in the 1980 election just a few scant years after Watergate tells you something about how comparatively easy it is for parties to rebuild themselves.
How bad is Trump, compared to Watergate? I don't know. But, parties can rebuild themselves. Diffuse support? That's hard to build, and if that gets torn down by batshit crazy allegations of the election being rigged, we don't know how to get it back.
Second, the fact that we haven't had revolutions is an indication of the level of diffuse support throughout the country. Make no mistake-- there are real dangers of violence on November 9. Trump is highly likely to lose, and he is spreading and magnifying batshit crazy conspiracy theories about why he is losing. That could lead to violence. Not an organized revolt, but it doesn't take organization to have one nutjob take a shot at another. However, 2016 is a sort of empirical test of our level of diffuse support. How much do Trump Republicans really believe in our constitutional system? We'll find out starting on November 9 if we see a peaceful transfer of power.
Finally, can a party break apart for non-policy reasons? Empirically, the last major party to break apart was the Whigs. That was over slavery. Every time a political scientist is asked about the potential for a party collapse, we go back to the Whigs and talk about the necessity for a cross-cutting issue that divides the party. While trade and immigration divide the Republican Party, and Trump is associated with those issues, that wouldn't be the thing that could break the party apart. If there is a risk of Trump breaking apart the Republican Party, it would be this: Trump loses, and blames the party establishment for not backing him. His voters decide they have been betrayed, and everyone takes sides in a war of personalities that dissolves the party. Can that happen? Probably not. But, right now, we can't rule anything out, and that brings me back to the title of the post. Parties can be rebuilt.
At this point, we should default to the prediction of Clinton winning, some sort of mess after the election, and the Republican Party eventually rebuilding itself. How bad will the mess be? Will there be violence? At this point, I have no idea. I don't rule anything out.