Photo by Ruth Fremson of The New York Times/Candidates for the Democratic nomination for president debate at Texas Southern University in Houston on Sept. 12. From left: Sen. Amy Klobuchar, D-Minnesota; Sen. Cory Booker, D-New Jersey; Mayor Pete Buttigieg of South Bend, Indiana; Sen. Bernie Sanders, I-Vermont; former Vice President Joe Biden; Sen. Elizabeth Warren, D-Massachusetts; Sen. Kamala Harris, D-California; the entrepreneur Andrew Yang; former Rep. Beto O'Rourke of Texas; and former Housing Secretary Julian Castro.

Before the last Democratic debate in November, party leaders will tighten the polling and fundraising criteria that candidates must meet to extend their participation. The political Grim Reaper will then eliminate more of the original aspirants.

The debates so far have allowed only bite-sized vote pitches. But in the next one, it would be helpful if candidates focused less on the minutiae of various proposals and more on the core principles upon which they would rely to provide the leadership that America needs and the presidency demands.

Previous presidents' ability to lead were tested by issues that, in retrospect, were far less serious than what we can expect. The next president will take office at a time when the nation seems adrift in the midst of uncertainty. For the first time in our history American democracy is at risk of cyber and social media attacks more sophisticated than in 2016.

That being the case, we need assurances of how the president proposes to confront those dangers, thereby ensuring America's survival and its mission in a hostile world.

Given the opportunity, I would urge the candidates to read a valuable new book, "Leadership in Turbulent Times," in which historian Doris Kearns Goodwin describes four men whose presidencies enabled the United States to survive terrible circumstances while simultaneously strengthening its system of government.

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Michael Loftin

Goodwin focuses on two Republicans and two Democrats — Abraham Lincoln, Theodore Roosevelt, Franklin Roosevelt and Lyndon Johnson — all of whom guided the nation fraught by desperate circumstances with the leadership necessary to resolve their respective challenges.

The book is divided into three parts: "Ambition and the Recognition of Leadership," "Adversity and Growth," and "The Leader and the Times: How They Led." The chapters cannot be easily summarized, but Goodwin's enlightening foreword will whet your appetite for the drama of each president's success.

Goodwin presents several important questions: Are leaders born or made? How does adversity affect the growth of leadership? Do the times make the leader or does the leader shape the times? Is leadership possible without a purpose larger than personal ambition?

"No single path carried [the men] to the pinnacle of political leadership," she writes. "Theodore and Franklin Roosevelt were born to extraordinary privilege and wealth, while Lincoln endured relentless poverty and Johnson experienced sporadic hard times."

Yet despite personal differences, "they were endowed with qualities often ascribed to leadership — intelligence, energy, empathy, verbal and written gifts, and skills in dealing with people." They were united by ambition and a drive to succeed, "and with perseverance and hard work, they made themselves leaders."

The men Goodwin profiles suffered personal adversities. Lincoln experienced near-suicidal depression. Theodore Roosevelt's young wife and mother died on the same day. Franklin Roosevelt's life was complicated by polio-induced paralysis. And despite his earlier successes, Johnson's 1941 loss in a Senate race nearly caused him to abandon politics altogether.

Yet all four "entered the presidency at moments of uncertainty and dislocation in extremis": for Lincoln, the Civil War and the emancipation of slaves; for Franklin Roosevelt, shepherding the nation during the Depression and a world war. Theodore Roosevelt and Johnson assumed office following the assassination of their predecessors.

By the time each man reached the White House, Goodwin writes, and "when guided by a sense of moral purpose, they were able to channel their ambitions and summon their talents to enlarge the opportunities and lives of others."

The presidents' actions on slavery (Lincoln), corruption (Theodore Roosevelt), the Great Depression (Franklin Roosevelt) and civil rights (Johnson) testified to the value of their leadership — and their willingness to do for others what their constituents were unable to do for themselves.

Goodwin shows how, regardless of their disparate personal histories, all four men were guided by a compulsion to serve those whom they were privileged to lead.

Faced with opportunities and crises, they willingly negotiated across party lines, eliciting support through persuasion, guile and, where necessary, pressure. The description of how Franklin Roosevelt deployed new programs and agencies as weapons against the Depression, for example, shows a willingness to swiftly replace an unworkable agency with another better suited to help Americans survive a disaster that had upended their lives.

The other three responded in similar ways to their respective crises: Lincoln issued the Emancipation Proclamation that began dismantling slavery. As governor of New York and later president, Theodore Roosevelt relentlessly prosecuted corruption. Johnson secured passage of two major civil rights laws.

Centuries ago, in I Corinthians 14:8, the Apostle Paul asked, "For if the trumpet makes an uncertain sound, who will prepare for battle?"

The verse's relevance to Goodwin's biography is obvious but not in a military sense. If a president's "leadership" is feckless, if it changes from one day to the next, thus sowing confusion and doubt, then serious consequences are inevitable. Families will worry about their future. Corporate planning will become more difficult. Military leaders will warn that our enemies will be emboldened by the uncertainty.

The presidency is not a vanity endeavor. The future of millions of Americans are inextricably linked to how a president leads — or worse, misleads — the nation. A strong ego, it can be argued, is crucial for anyone who ascends to the presidency.

Humility is equally important. In her examination of the four presidents, Goodwin notes that they were able to succeed even as they were humbled by the enormity of presidential responsibilities.

That said, a quotation often incorrectly attributed to C.S. Lewis is instructive:

"Humility is not thinking less of yourself. It is thinking of yourself less."

Michael Loftin is a former opinion page editor of The Chattanooga Times.