In his appearance before the House Oversight Committee in late February, President Trump’s former fixer Michael Cohen saved his most disturbing words for his concluding statement, when he said he fears that if Trump loses the 2020 election, “there will never be a peaceful transition of power.” Cohen did not elaborate, and the committee members had no opportunity to pose follow-up questions. But imagine, for example, a catastrophic terrorist attack that prompted Trump to declare a national emergency with the support of congressional Republicans, the conservative majority on the Supreme Court, and Fox News, Breitbart, and countless websites. In such circumstances Cohen’s scenario is not utterly inconceivable.

In If We Can Keep It, Michael Tomasky raises no alarms about the prospects of a Trumpian coup, but like Cohen he believes that the republic is deeply imperiled. For Tomasky, though, Trump and his supporters are not the sole or even the biggest threat. “Chapter for chapter,” he writes, “most of this book could have appeared just as it now stands no matter who became president.” He is more troubled by the partisan polarization and consequent dysfunction that made Trump’s presidency possible.

In accounting for the dysfunction, Tomasky sometimes offers a version of today’s blander political punditry, which holds that we have become polarized because the middle has fallen out of American politics. His formulation speaks of the decline of what he calls “intraparty polarization”—that is, of the strong differences inside the major parties that have traditionally tempered polarization between the parties. As both parties have become more ideologically homogenous, he observes, opportunities for cross-party alliances have vanished; “now,” he writes, “we have party tribalism.” Caught up in that tribal warfare, “more and more Americans”—those whom pundits are constantly pointing to as the missing middle—“are turned off by parties.” Meanwhile, in a vicious spiral, both sides become ever more fervent in demonizing their adversaries.

One weakness of this kind of formulation is that it substitutes a description of current divisions for an analysis of how they came about. To assert that parties are more polarized today because they are no longer the broad coalitions they once were is an improvement over simply saying that the center has disappeared, and it adds useful historical insight, but it still comes very close to being tautological. By implication, it also holds Democrats as well as Republicans responsible for the collapse of political cooperation, if not necessarily to the same degree. The conventions of faux objectivity demand finding fault with both sides for today’s polarization, if only to avoid being dismissed as partisan, and when Tomasky pays lip service to those conventions, even faintly, it weakens his analysis. At his strongest, though, he forthrightly indicts the Republican Party as the chief malefactor in creating and exploiting the country’s divisions.

Numerous previous commentators, from divergent political positions, have contributed to the…