LONDON — During an interview in late 2016 about the rise of populism, Yascha Mounk, a lecturer at Harvard University who is the author of the new book “The People vs. Democracy,” made an offhand comment that now seems grimly prophetic.

In recent years, he said, Europe’s center-left and center-right parties had tended to work together, either unofficially or in a grand coalition, in what was viewed as high-minded and sensible, an important bulwark against the rise of the far right.

But Mr. Mounk believed such cooperation ultimately led to instability. Those “ideologically weird” coalitions tend to alienate voters, he said, by implying that there was little real difference between establishment right and left.

And grand-coalition politics, he said, also meant that “the things that the populists have said all along — that everything else in politics is the same and a conspiracy against them, and the only way to change the government is to vote for the populists — become true.”