JAMES TARANTO INTERVIEWS MORRIS FIORINA: Moderate Voters, Polarized Parties.

Most observers of American politics predict 2018 will favor the Democrats. The party has a good chance of taking control of the House in November, and even a Senate majority is within reach, although Democrats are defending three times as many seats in the upper chamber as Republicans are.

Here’s a safer prediction: If the Democrats do triumph on Nov. 6, they and their supporters will emerge triumphalist, proclaiming their majority permanent and President Trump a lame duck. Ten months in advance, Morris P. “Mo” Fiorina has a bucket of cold water to throw on such claims.

Mr. Fiorina—no relation to 2016 presidential candidate Carly Fiorina or her husband—is a 71-year-old Stanford political scientist and author of a new book, “Unstable Majorities: Polarization, Party Sorting and Political Stalemate.” As the title suggests, he believes the U.S. has entered an era in which no party can hold a majority for very long. “We can change our pattern of government every two years,” he tells me on a recent visit to the Journal’s offices, “and we started doing that.”

Did we ever. The party controlling the House, Senate or White House changed in seven of the nine elections between 2000 and 2016—the only exceptions being the presidential re-election years, 2004 and 2012. “I sort of trace it back to ’92, the end of the Republican presidential era, and then ’94 is the end of the Democratic congressional era,” Mr. Fiorina says.

Hmm.