David Shribman is the former executive editor of the Pittsburgh Post Gazette.

There is a tide in the affairs of America, and the big question is whether right now the political seas are at ebb tide or flood tide.

American politics is guided by these tides, and in retrospect, their movements become clear. Future students of history may come to see that contemporary America was shaped by two of them, each about four decades long. Their precise beginnings and ends are murky, but the general shape of them — the swells and the troughs — may be apparent when mid-21st-century historians begin to examine what we now regard as lived experience and modern history. For them, all this may seem like ancient history.

The first of these tides might be said to have started in 1932 and almost certainly will come to be seen as a great Democratic era, populated by giants (Franklin Delano Roosevelt, Harry Truman, John F. Kennedy, Lyndon B. Johnson) and marked by the conquest of both the Great Depression and the Second World War; a great expansion in the role of government; and soaring rhetoric about ending injustice and smoothing out the rough edges of life.

The second tide might be thought of having begun in 1976 and may be seen as a dominant Republican era, populated by a powerful mixture of establishment figures (Richard M. Nixon and George H.W. Bush) and rebels (Ronald Reagan and Donald Trump), and marked by the conquest of Soviet-style Communism; by a fundamental rethinking of the role of government in everyday life and in the economy; and by a celebration of the philosophy and arts of entrepreneurship.

What ties them together — what ties together almost a century of American life — is the conviction, held dearly by political figures in both parties, that politics is the expression of upswells of public will. Even though the rhetoric in both waves focused on the middle class — its growth and shrinkage, its prospects and perils — both tides realized their power through the welfare, interests and inclinations of blue-collar Americans.

The animating conviction was, as Mr. Kennedy might put it, that a rising tide — whether because of the new opportunities that accompanied new rights or the liberation that came from the removal of government interference in the economy — lifts all boats.

And so the shifting loyalties of what we might call “working America” — not middle-class Americans nor soccer moms in the suburbs, important as they have been in stump speeches — are the moving parts of the country’s politics.

That was true in the 19th century, as the elections of Andrew Jackson (1828 and 1832) and William Henry Harrison (1840) displayed. It began to coalesce in modern times with the unsuccessful 1928 campaign of Democratic Gov. Al Smith of New York and was consolidated with the election of FDR in 1932.

Under Mr. Roosevelt and the Democrats who followed the New Deal with the Fair Deal (Mr. Truman), the New Frontier (Mr. Kennedy), the Great Society (Mr. Johnson) and the New Covenant (Bill Clinton), rights were expanded and horizons were broadened.

The first two years of the Nixon presidency can almost be regarded as an extension of the Democratic wave, with the creation of the Environmental Protection Agency and with the administration winning passage of a landmark income-floor plan in the House but failing to win support in the Senate. It was the Nixon team’s pillorying of a similar 1972 plan forwarded by Sen. George McGovern — a flip from Nixon approval of the plan to opposition to it — that began the transformation from one era to the next.

The following two years were consumed with Watergate. Then came the second wave.

That wave commenced with the insurgent 1976 presidential campaign of former Gov. Reagan of California, who mounted an outsider’s challenge to an establishment incumbent president, Gerald Ford, who had served 25 years in the House, always as a supplicant in a chamber controlled by the Democrats.

From that moment, the interesting story of American politics moved to the Republican side.

Not that the Democrats were denied successes in that period. Jimmy Carter rode the Watergate counter-reaction to the White House; Mr. Clinton served two terms; Barack Obama become the first Black president. But in that period, the Democrats were a faint, unimaginative variation on an old theme, while the Republicans fought to define themselves even as they succeeded in defining the era.

The Reagan insurrection was followed by years of struggle over the role of religious conservatives in the GOP, a proxy for an important battle over the role of religion in the public square. The anti-abortion movement emerged from that, with the 1988 GOP national convention convulsed by a class struggle, a power struggle and a political struggle.

Those struggles persist to this day. “The key question of the future of American politics,” the late Brookings Institution fellow A. James Reichley told me as that convention opened, “is how the Republican Party handles these tensions.” He was right.

An incipient trend that began in the Nixon years — the creation of a “Democrats for Nixon” effort — eventually became one of the major themes of the remainder of the 20th century and the first quarter-century of the 21st. By the time Mr. Reagan won the 1980 GOP nomination, the trickle of onetime Democrats, most of them blue-collar workers but a few intellectuals (such as Jeane Kirkpatrick and other “neocons”), into the Reagan circle became a rivulet, then a flood. This movement presaged the insurgency of Mr. Trump, himself a onetime Democrat who spoke to the grievances of blue-collar voters and sculpted a new profile for the GOP — essentially the reverse of the New Deal coalition.

Now we may be at another turning point, and, like so much else in American politics, Mr. Trump is at the center of it. Note that we cannot place Joe Biden in that role; he’s a purebred product of the first wave and is in our politics only because of longevity and because he was the candidate who denied Mr. Trump a second term.

If Mr. Trump is elected, the Republican wave will continue, at least for another four years, and the children and grandchildren of New Deal Democrats may grow ever more comfortable in their GOP raiments.

Almost everyone — members of both parties, though for vastly different reasons — believe the future of democracy is at stake in 2024, and they are probably right. But so is the nature of our democracy.