FOR instance, Democrats are more loath than Republicans to look squarely at the government debt crisis indisputably looming with the aging of baby boomers and the ballooning cost of Medicare. Republicans are more reluctant than Democrats to acknowledge the rise of global temperatures and its causes and consequences. But both parties, it is fair to say, prefer not to consider either trend too deeply.

Both parties would rather avert their eyes from such difficult challenges — because we, the people, would rather avert our eyes. Talk to any political pro about this phenomenon and one name inevitably comes up: Jimmy Carter, who has become a sort of memento mori for American politicians, like the skulls in Renaissance paintings that reminded viewers of their mortality.

Mr. Carter, they will say, disastrously spoke of a national “crisis of confidence” and failed to project the optimism that Americans demand of their presidents. He lost his re-election bid to sunny Ronald Reagan, who promised “morning in America” and left an indelible lesson for candidates of both parties: that voters can be vindictive toward anyone who dares criticize the country and, implicitly, the people.

This is a peculiarly American brand of nationalism. “European politicians exercise much greater freedom to address bluntly the uglier social problems,” says Deborah Lea Madsen, professor of American studies at the University of Geneva. An American politician who speaks too candidly about the country’s faults, she went on to say, risks being labeled with that most devastating of epithets: un-American.

The roots of this American trait are often traced to the famous shipboard sermon the Puritan lawyer John Winthrop preached on his way to help found the Massachusetts Bay Colony nearly five centuries ago.

“We must consider,” he said, “that we shall be as a city upon a hill — the eyes of all people are upon us.” Winthrop’s metaphor has had a long life in American speechifying, prominently quoted by both President John F. Kennedy and Reagan. But if, for Winthrop, the image was something the colony should aspire to, for modern politicians it is often a boast of supposed accomplishment, a way of combating pessimists and asserting American greatness, whatever the facts.

Could a presidential candidate today survive if he promised to wage a war on poverty, as President Lyndon B. Johnson did in 1964? It seems unlikely, and one reason may be that Johnson’s effort fell short, revealing the agonizing difficulty and huge cost of trying to change the lives of the poor.