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

On one side is a lifetime politician who has spent almost a half-century in elected office; on the other side is a relative political newcomer who disdains the political experience that his rival has accumulated.

On one side is a leader who has respect for the conventional practices of politics; on the other is a leader whose entire identity is wrapped up in destroying the conventional.

On one side is an elderly man who believes that past practices are adaptable to contemporary circumstances; on the other is an elderly man whose campaign is based on defiance of past practices.

On one side is a political veteran who by virtue of longevity and, now, inclination, is an establishment figure; on the other is an outsider who has contempt for the inside game.

Not for a century and a quarter has the United States seen a presidential election remotely like this one.

And the irony of this collision — the peculiar, defining element of this election — is that both contestants have won the White House in the past.

Joe Biden, born at the end of the Silent Generation, and Donald Trump, born in the very first year of the baby boom, may be only 3 1/2 years apart in age, but they are from different worlds entirely.

While Franklin Delano Roosevelt was an urban figure and Alf Landon’s identity grew from the oil fields of the Kansas plains, the two competitors in the 1936 election — possessed of vastly different political views — were both governors and built their political identities and styles on those of Theodore Roosevelt. While John F. Kennedy, Harvard ‘40, was reared in privilege, and Richard Nixon, Whittier College ‘34, grew up in hardscrabble circumstances, the two 1960 competitors — possessed of sharply divergent visions of the country — were both Navy lieutenants in the Pacific in World War II and had served in both the House and Senate.

George H.W. Bush was a Connecticut patrician, the son of a Wall Street banker and senator, and he enlisted in the Navy. Bill Clinton grew up in Hot Springs, Arkansas, was reared by a single mother and his grandparents, and maneuvered to avoid the Vietnam-era draft. But both had elite educations, international travel experience and mainstream political views. George W. Bush was an indifferent student who came to politics late in his life. Al Gore was a serious student who was elected to the House of Representatives at age 28. But both 2000 nominees were sons of politicians, and both of their fathers were Senate candidates in 1970.

Biden and Trump have no such intersections. The Venn diagrams of their lives have no overlap.

Biden went to a private Catholic high school, Trump to a military academy. Biden is a lifelong liberal with strong roots in the Democratic Party; Trump has been associated with both parties and has had a fluid ideology. Biden cultivates the image of a cool guy at ease in a Corvette Stingray; Trump poses as a real-estate warrior in a Fleetwood Cadillac.

Those who served with Biden earlier in his career might be surprised that he is in the presidency at age 81 — he first ran for the White House at age 45 and flamed out. Those who grew up with Trump would be astonished to learn that their classmate had reached the presidency, though their memories of a boy who slapped around others and sought to dominate them would resonate with Americans today.

The factor that sets this election apart is that one of the contestants is a former president who, despite four years in the Oval Office and decades in business, is an insurgent.

Indeed, there hasn’t been an insurgent as a presidential nominee of a major political party since 1896, when the Democrats — then a party of small government with trace elements of post-Reconstruction racism that was only beginning to become a magnet for immigrants and the striving — nominated William Jennings Bryan for president. Like Trump, he would eventually win three nominations.

Bryan’s ascendancy occurred as the Democrats co-opted the Populists, a prototypical third party but one that had won more than a million votes in carrying five states in 1892. In his classic 1964 examination of the 1896 election, “McKinley, Bryan, and the People,” the historian Paul W. Glad noted that those who rushed to Bryan’s standard were moved by the wealth gap of the time and by the conviction that American economic inequality had been built on the foundation of industrial capitalism. “They compared the advantages of the rich with the hardships of the poor,” Glad wrote, “and they urged a program which they thought would establish social and economic justice.”

The targets of that late-19th century appeal were farmers — an important voting block at a time when, according to the Census Bureau, only 35% of the population lived in urban areas.

A Colby College poll showed that two-thirds of rural Republicans supported Trump in this year’s primary season, that a large majority of rural voters haven’t seen any impact of Biden’s economic policies, and that rural Americans were more likely to consider the economy to be in danger.

In Bryan’s stump speeches, he said that “no government is worthy of the name which is not able to protect from every arm uplifted for his injury the humblest citizen who lives beneath the flag.” Trump’s speeches hit many of these same notes. In his first presidential race, he told a rally in Ambridge, Pennsylvania, whose steel works contributed to the Bay Bridge in California and the Brooklyn Bridge in New York, “I consider myself in a certain way to be a blue-collar worker.”

“Both Bryan and Trump speak in a distinct populist rhetoric, and both push the idea there’s an elite that’s un-American,” said Michael Kazin, the Georgetown University historian who is the author of the 2006 book “A Godly Hero: The Life of William Jennings Bryan.” “Trump is a celebrity, and he is a great performer, and so was Bryan. Until he died, Bryan could draw a huge crowd wherever he went; it was said that he was good for several rows of Ford Model Ts every time he gave a speech.

“But,” Kazin said, “the difference is that Trump was president, and Bryan never really had a realistic chance of being president.”