What makes a good president? It's a question of perennial interest—and one that may seem particularly relevant (or ironic) just after the Republicans and the Democrats have held their national conventions. Historians don't have access to infallible criteria. But at least they have the perspective of time. They can judge reality against campaign promises. In the middle of an emotionally charged campaign, two reasonably intelligent voters can believe that one candidate will make America great again and the other person's choice will send the country into irreversible decline. You can be sure that Donald Trump or Hillary Clinton would ruin America as president. But only historians claim the authority to rank someone alongside James Buchanan and Warren Harding.

How, then, do historians judge the 44 US presidents? One interested observer to this debate said, "The mark of a leader is whether he gives history a nudge." When ranking presidents, this comment certainly applies to such remarkable executives as Thomas Jefferson, Abraham Lincoln, and Franklin Delano Roosevelt. But it also applies to the man who made the statement: Richard Nixon. He nudged the United States toward relations with Communist China, and also nudged the American population toward cynicism when Watergate revealed his true character.

More conservative observers might urge great presidents to affirm, "The history of liberty is the history of the limitation of governmental power, not the increase of it." This statement sounds like one of the Founding Fathers, perhaps George Washington or John Adams. Or maybe that most photogenic president, Ronald Reagan, offered these words to support his case that government is the source of our problems. Actually, it was Woodrow Wilson, who by the end of his second term advocated, unsuccessfully in his own country, for a League of Nations.

History, in the eyes of those who write about it, belongs to the doers, the leaders who nudge the world toward progress. Such a standard would seem to benefit liberals, who earn extra credit for expanded constitutional rights and government programs. But the great conservative presidents, such as Lincoln and Reagan, nudged history themselves as commanders-in-chief during times of war, whether civil or cold. Indeed, everyone near the top of most rankings has expanded government or fought a war or both.

Even so, when viewing the presidency through the historian's eyes, you can see why the country tends to move in a liberal direction. The modern media campaign demands a purpose, a plan. How do you run for an office you don't intend to use? Why command an army if you don't want it to fight? And once a program has been started, it's nearly impossible to take it away. When presidents launch wars, they tend to unleash liberal forces far beyond anyone's control. Nothing is less conducive to conserving the status quo or budgetary constraint than mass military mobilization. For example, Wilson's government, committed to peace through his re-election, spent more money in 19 months on World War I than the entire federal government had spent on everything combined until then. But that conflict pales in comparison to World War II, which cost more than everything the government spent between 1789 and 1940.

When assessing a president, journalists need copy. They'll write either about your accomplishments or your scandals. And when historians read those contemporary accounts, they need narratives. They need action. That's why Calvin Coolidge belongs to the era of the silent film, relegated to the dusty attics of forgotten history. He lacked both personality and policy. To rank among the greats and earn a PBS miniseries from Ken Burns, you need both.

William E. Leuchtenburg, author of The American President: From Teddy Roosevelt to Bill Clinton, is professor emeritus at the University of North Carolina in Chapel Hill. He is an expert by virtue of both productivity and also longevity. He was born when Harding was president, and he has written or edited eight books about FDR and his era. He seeks in this book to advance his thesis that the modern, active presidency did not begin with FDR but with his older cousin, Theodore Roosevelt, who succeeded the assassinated William McKinley in 1901. So this book that spans the entire 20th century hinges not on an arbitrary date but on a dramatic transformation in what we as Americans want and demand from our presidents.

No doubt the book's length will turn away many readers. That's a shame, because Leuchtenburg actually writes with flair. He includes no footnotes, and when you consider how much happened in a century that ranged from the Great War and the Great Depression to Watergate and Whitewater, not even an account of this length allows for more than a brisk tour in which Leuchtenburg offers largely orthodox interpretations of his widely studied subjects. Still, The American President provides perspective only available from the best historians, who help us remember the past as it was and not as nostalgia would have it. Rather than memorializing the 1950s as an interval of peace and prosperity, for example, Leuchtenburg describes the decade as the "most terrifying the country has ever experienced." It's hard for those who didn't live through the 1950s to remember it as a time dominated by the looming threat of nuclear annihilation.

The president most closely associated with the 1950s, Dwight Eisenhower, doesn't usually rate highly among historians, though his stock has been slowly rising. After all, you don't get credit for what doesn't happen. Under Eisenhower the country enjoyed few new rights and few new government programs, at least when compared to the bookend 1940s and 1960s. And the great victor of World War II did not lead the nation into World War III after the Korean conflict ended. The last president born in the 19th century, Eisenhower reflected some of that era's assumptions, when presidents did not regard it as their responsibility to provide for the unemployed during economic downturns or engineer other social reforms. The electorate did not expect them to do so. Until FDR in the mid-20th century, few Americans bothered to write the White House, because they did not expect such a personal connection with the president. Teddy Roosevelt was ahead of his time when he said, "Better the occasional faults of a government living in the spirit of charity than the consistent omissions of a government frozen in the ice of its own indifference."

Perhaps the greatest insights from Leuchtenburg come in his masterful if reductionist sense for deploying illustration to capture the mark of a man. Describing the vanity of LBJ, he writes, "When he visited the Vatican, the pope gave him a Renaissance painting. In return, Johnson presented His Holiness with a bust of himself." Explaining the unprecedented influence of First Lady Hillary Clinton, he tells a story of when Chelsea Clinton needed parental permission at school to take a pill. The administrator offered to call her mother. "Oh, don't call my mom," Clinton said. "She's too busy. Call my dad."

Bill Clinton was only the last in a long string of 20th-century presidents whose second terms were undone by scandal. Leuchtenburg's explanation for this discouraging phenomenon comes earlier in words about Teddy Roosevelt: "If Roosevelt showed the value of a strong president who battled for the people, he also demonstrated the danger of unbridled executive power." By the end of the 20th century, the presidency hardly resembled what the Founding Fathers intended. "Though the Founding Fathers had taken pains to avoid creating another George III when they framed the presidency," Leuchtenburg writes, "the office in the twentieth century took on aspects of the majesty, even divinity, that doth hedge a king."

From Roosevelt to Clinton we see that the more power we give the president, the less we trust him. The higher our expectations rise, the greater our disappointments. We're not content with a chief executive. We want someone who moves the emotional register of the nation with the optimistic confidence of Ronald Reagan and the heartfelt empathy of Bill Clinton. So our presidents get credit for things they don't do and blame for things they don't control. We want our presidents strong like Clinton when they stare down the likes of Newt Gingrich in conflict over a government shutdown. But they can't handle their power, as when amid that high-pressure shutdown Clinton began his infamous affair with Monica Lewinsky.

Hence we vacillate between unrealistic hopes and abject cynicism, as did one American who gave up voting in 1976. "I'm a three-time loser," the erstwhile voter said. "In 1964 I voted for the peace candidate—Johnson—and got war. In '68 I voted for the law-and-order candidate—Nixon—and got crime. In '72 I voted for Nixon again, and we got Watergate."

So what makes a good president? Maybe we need to change our expectations. "We give the President more work than a man can do, more responsibility than a man should take, more pressure than a man can bear," the novelist John Steinbeck said. "We wear him out, use him up, eat him up … . He is ours and we exercise the right to destroy him."

Collin Hansen is the editorial director for The Gospel Coalition, an editor at large of Christianity Today, and the author most recently of Blind Spots: Becoming a Courageous, Compassionate, and Commissioned Church (Crossway).

Copyright © 2016 by the author or Christianity Today/Books & Culture magazine.
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