What if Republicans win the midterms?

For a counterexample of how a time of intense political bitterness can start to tear this country apart, look back exactly half a century to 1968. In that chaotic year, America slashed and clawed at itself amid the assassination of the Rev. Dr. Martin Luther King Jr. and the riots that followed; the assassination of Robert F. Kennedy as he ran for president; swelling antiwar demonstrations on college campuses amid growing recognition that the government had been lying about the course of the Vietnam War; and the police beatings of protesters in Chicago outside the Democratic National Convention.

Factors like the draft and the race relations of the period made that tumultuous year a particular historical moment. But one difference between then and now is salient: Arguably, there was little reason to believe that the November 1968 election was likely to provide immediate relief.

We are lucky that so far 2018 does not look like a new 1968. But the relative calm may be like an unexploded bomb, its volatility not so much defused as contained by the thought that Trump Republicans will be punished in the Nov. 6 midterm elections. These expectations are widespread. After the big Democratic special election victories in places handily carried by Mr. Trump in 2016, from Virginia and Alabama to Wisconsin, Republican lawmakers in purple districts are retiring to avoid ending their careers in humiliating defeats.