Despite his reputation as a disrupter, Donald Trump has not been able to break the political stalemate afflicting America for half a century.

Since 1968, neither major political party has been able to command an enduring electoral majority. Such stasis is unusual in American politics, if one can call unusual something that has been happening for 50 years. Still, the older pattern, now almost forgotten though still longed for by strategists of each party, was quite different. To draw the most striking contrast, in the 72 years between Abraham Lincoln’s election in 1860 and Herbert Hoover’s crushing loss in 1932, the Republican Party controlled the presidency for all but 16 years. Only Grover Cleveland and Woodrow Wilson interrupted the GOP’s electoral serenity.

In turn, the Democrats began their own reign, holding the presidency from 1932 to 1968, with the exception of Dwight Eisenhower’s two terms in the 1950s. That’s eight years of a Republican chief executive to 28 for the Democrats. But Ike had been courted by the Democrats before he agreed to run on the GOP ticket, and his agenda of “Modern Republicanism” stressed its continuity with New Deal foreign and domestic policy. So the era seemed even more unrelievedly Democratic than the presidential numbers would suggest.

Add to that the Democratic Party’s lopsided control of Congress in those years—and beyond—and the full picture emerges. In the 36 years between Franklin Roosevelt’s roaring entrance to, and Lyndon Johnson’s meek exit from, the White House, Republicans enjoyed control of the U.S. House of Representatives and the Senate together for (ta-da) four years (1946-48, 1952-54). The Democratic sway over the House that commenced in 1954, by contrast, would last until 1994, an unprecedented 40 years’ control by the same party. It was a similar story in the Senate, held by Democrats from 1954 to 1980, when Ronald Reagan helped to pry it from their grasp.

For most of the century between 1860 and 1968 (the big exception is a 20-year period at the end of the 19th century), the dominant party enjoyed control simultaneously of the presidency, the Senate, and the House—“undivided government,” as the political scientists call it. That pattern, a corollary of the stable dominance of a national majority party, has gone the way of the dodo since 1968.

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In our era, voters are fickle, never trusting either party with undivided control for very long, and more or less alternating the parties’ hold on the presidency. Here is the list: eight years of Republican Presidents Nixon and Ford, four years of Democrat Jimmy Carter, 12 years of Reagan and Bush the Elder (R), eight years of Bill Clinton (D), eight of Bush the Younger (R), eight of Barack Obama (D), and three, so far, of Trump. That’s 31 years of GOP presidents, and 20 of Democratic ones—a Republican edge, to be sure; but the more impressive fact is the sustained alternation between the parties. In the past half century, the voters have experimented with undivided government for only 14 years, eight under the Democrats and six under the GOP, and never for longer than four years in a row. (I don’t count the confusing year after the close 2000 election, when George W. Bush had, and then lost within months, a one-vote Republican margin in the Senate.)

Mr. Trump succeeded a president of the other party, came into office with his fellow Republicans in control of both the House and Senate, promptly lost that shot at undivided government in his first midterm election (in 2018, even as Obama did in his first midterm in 2010), has faced a torrent of criticism and Resistance doubting his legitimacy and fitness for the office, and now faces a difficult reelection race. In all of these particulars he fits squarely into the larger patterns of post-1968 American politics.

His nationalism, “populism,” brusque way with subordinates and allies, addiction to Twitter, love of tariffs, criticism of illegal immigrants, disdain for political correctness—these and the several other features that might be said to be distinctive of it have not, so far, boosted his presidency out of the usual political orbits or changed the overall trajectory of American politics, at least in its electoral dimension.

Even in facing impeachment, he confirms another disturbing trend of modern politics. In the first 185 or so years of the republic (until 1974), Americans impeached one president. In the past 45 years (since 1974), we have impeached or come close to impeaching three (Nixon, Clinton, Trump).

Despite what his detractors and even some of his admirers say, Donald Trump is a normal president for our times. And so far, at least, the times are not a-changin’.