For a variety of reasons, the nomination of Donald Trump would probably not be in the best interest of the Republican Party. Such an outcome this year, which seems increasingly likely, would either imply that the book’s hypothesis was wrong all along — or that the current Republican Party is weak and dysfunctional and perhaps in the midst of a realignment.

You might associate “The Party Decides” with an empirical claim made in the book: Endorsements made by influential Republicans and Democrats are a good predictor of who will win each party’s nomination. At FiveThirtyEight, we’ve been keeping track of a subset of those endorsements, those made by current governors and members of Congress in each party.

Our focus on endorsements by governors and members of Congress is mostly a matter of convenience, however. In “The Party Decides,” the authors consider a much broader array of endorsers, including state legislators, labor unions, interest groups and even celebrities. This is important because, in contrast to earlier scholarship that thinks of parties as consisting solely of politicians and party organizations like the Republican National Committee, the authors of “The Party Decides” take a more inclusive view. Their parties include not just elected officials but also “religious organizations, civil rights groups … organizers, fundraisers, pollsters, and media specialists” and even “citizen activists who join the political fray as weekend warriors.”