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

Let’s return for a moment to the two conventions now concluded and consider one of the important things that didn’t happen — and what that omission tells us about the two parties.

At the Democratic convention were Bill Clinton and Barack Obama, who together accounted for 16 years in the White House (1993-2001 and 2009-2017), along with the grandsons of John F. Kennedy (1961-1963) and Jimmy Carter (1977-1981), who added another six years; Hillary Clinton, a onetime Democratic nominee (2016); and Joe Biden, the current president.

Weeks earlier, George W. Bush, a Republican who won two terms in the White House (2001-2009), and Mitt Romney, a sitting senator (from Utah), a successful governor of a blue state (Massachusetts) and the party’s presidential nominee (2012), were not at the Republican National Convention.

This may be a chapter from a Peter Whalley novel and BBC radio thriller. Its title is “The Absent Guest.”

This isn’t a case of absence makes the heart grow fonder. Instead, these absences are making the party go further in solidifying its identity as the personification of former President Donald Trump’s political views, profile and tactics.

Both parties traditionally have welcomed their living presidents and many of their past nominees to their conventions. That represents more than a gesture of respect; it’s a signal that American political parties are more than platforms for contemporary political battle; they also are repositories of American history. The younger of them, the Republican Party, was founded 170 years ago and is justly proud of its sobriquet, the Grand Old Party, so prominent that the initials “GOP” require no explanation.

The effort to display a glorious political past propelled the Democrats to place giant photographs of Kennedy, Harry Truman, Franklin D. Roosevelt and Lyndon B. Johnson over the rostrum at the party’s 1964 Atlantic City, New Jersey, convention, along with a huge poster proclaiming the slogan “Let us continue ...”

In the proceedings just past, Obama said Trump’s style has “gotten pretty stale,” adding, “We do not need four more years of bluster and bumbling and chaos. We’ve seen that movie before, and we all know that the sequel is usually worse.”

The next night, Clinton compared Vice President Kamala Harris to her opponent, saying that the election presented a clear choice: “We the People” versus “Me, Myself and I.”

The two presidents’ appearances were, to be sure, partisan rallying cries. But they also were homages to the Democrats’ values, symbols of the party’s illustrious past and suggestions that its future is part of a great sweep of continuity.

That’s why, in 1956, former President Truman stood before the Democrats’ Chicago convention and endorsed the idea of a major civil rights bill, which the Democratic-controlled Congress delivered the next year. He appeared on the podium with Sen. Estes Kefauver of Tennessee, who would become former Gov. Adlai Stevenson’s running mate; Gov. Averell Harriman of New York; Senate Majority Leader Lyndon Johnson of Texas; and House Speaker Sam Rayburn of Texas.

That’s why former President Ronald Reagan, speaking at the Republicans’ 1992 convention in Houston, told delegates that “we must be equal in the eyes of each other,” imploring in words that might be appropriate today, “We can no longer judge each other on the basis of what we are, but must, instead, start finding out who we are. In America, our origins matter less than our destinations, and that is what democracy is all about.”

That’s why Carter, speaking at the Democrats’ 2004 convention in Boston, said of his party, “We lack neither strength nor wisdom. There is a road that leads to a bright and hopeful future.”

And that’s why sitting presidents often appear at national conventions to give support to the nominees who hope to succeed them.

Having eschewed a reelection campaign in 1952, Truman proclaimed his support for Gov. Stevenson, setting forth the Democrats’ creed before the crowd in the International Amphitheatre in Chicago, saying, “We believe in an unlimited America. Unlike our Republican opponents, we would not limit our goals to the mere compass of our fears. Instead, we will extend our goals to match our hopes.”

In the same spirit, Clinton, who was ineligible for a third term, spoke at the Staples Center in Los Angeles in support of Vice President Al Gore, the party’s 2000 nominee. “We’ve worked closely together for eight years now,” he said. “In the most difficult days of the last years, when we faced the toughest issues — of war and peace, of taking on powerful special interests — he was always there.”

Biden gave a similar speech in support of Harris, employing almost exactly the phrasing Clinton applied to Gore in describing his selection of Harris as his vice president as “the best decision I made [in] my whole career.”

An earlier generation of conservatives revered Edmund Burke (1729-1797), who, among his many virtues as a thinker and British parliamentarian, was an advocate of the American Colonists’ critique of London’s imperial rule. In his monumental work, “Reflections on the Revolution in France” (1790), he wrote that society is a “partnership not only between those who are living, but between those who are living, those who are dead, and those who are to be born.”

The GOP absences represent a defiance of Burke by the party that for the past century has been the happy home of American conservatism.

Trump has criticized Bush for his “failed and uninspired presidency” and said that Romney, who voted to impeach Trump, was a “choke artist” who was “a failed candidate.”

The 45th president clearly didn’t want to be portrayed in a line of successful past presidents — or to have the affirmation and support of senior members of the party.

“The kind of appearances that didn’t occur at the Trump convention provide star power and a blessing — and show that the nominee is honoring the great traditions of the past,” Kevin Boyle, a Northwestern University historian, said in an interview. “The Republicans’ omission of this is a really dramatic departure. Republicans now are in specific rejection and, really, at outright war, with its past leaders. They’ve expunged the traditional Republican Party. This is a party that wouldn’t welcome Reagan as a speaker.”

The result of the November election might determine whether the 2028 Republican convention would welcome Trump.