What astonished them the most, Levitsky told me in an interview last week, "was the speed and the degree to which the Republican Party Trumpized." In "How Democracies Die," he and Ziblatt had reproved Republicans for failing to stop Trump's rise to power. But at the time, he said, "we didn't consider or call the Republican Party an authoritarian party. We did not expect it to transform so quickly and so thoroughly."

All liberal democracies have some countermajoritarian institutions to stop popular passions from running roughshod over minority rights. But as "Tyranny of the Minority" shows, our system is unique in the way it empowers a minority ideological faction at the expense of everyone else. And while conservatives like to pretend that their structural advantages arise from the judicious wisdom of the founders, Levitsky and Ziblatt demonstrate how many of the least democratic aspects of American governance are the result of accident, contingency and, not least, capitulation to the slaveholding South.

It can't happen soon enough.

Republicans have been openly authoritarian since 9/11.

I was thinking the very same thing. The Unitary Executive Theory was the functional genesis point IMO, and Cheney did a damn fine job of implementing it with his actions on 9/11 and subsequently as we invaded Iraq based on lies and misinformation.

shouldn't it be "underestimated "?

No, because the authors were thinking in the affirmative, ie., that Republicans actually cared about protecting and preserving our constitutional democracy the way they've always claimed through the years.

So in this context, "overestimated" would be correct.