not trying to perpetuate something unproductive here, but what's really new?

In the past, partisan identification and voting patterns were largely driven by positive partisanship. Voters that were true partisans would be allied to their preferred party because of positive support for their identified party. A lifelong yellow dog Democrat would be dedicated to their party because of their party - they might be convinced of the rightness of their policies, or derive patronage benefits from being affiliated with the party, or admire/esteem its leaders, or any of a bunch of other factors specific to their party. And the same would be true of a rock-ribbed Republican.

That's now been replaced to a large degree by negative partisanship. Partisan voters aren't supporting their party; they are opposing the other party. The die-hard Democratic voter isn't motivated by what they love about the Democrats, but what they hate about Republicans (and vice-versa).

This matters, because it affects how voters respond to parties' failures and problems. When voters are positively partisans, keeping their support requires keeping their love/esteem/well regard for your party. Democrats keep the Democratic base in their column by delivering them the things they love, and vice-versa for the Republicans. The degree of support that positive partisans will provide depends a lot on how they are feeling about their own party.

With negative partisanship, though, voters are far less concerned about what's happening within their own party - they're motivated by what's happening in the opposition party. It doesn't matter all that much whether their own party is doing something that's disappointing to them - they're motivated by resistance to the other guys, after all. As long as you're stopping the other guys from winning, you're meeting the needs of those voters.

For negative partisans, their candidate isn't all that central to their decision-making. What matters is the other candidate. Which is why, I think, this explains why Trump has such a high floor despite all of his many faults. Those faults are beside the point, except to the extent that they make it harder for him to stop the Democrats. Because stopping the Democrats is what's necessary to help the country, irrespective of whether Trump is a terrible person or not.