WASHINGTON » Imagine a presidential candidate who spoke with blunt honesty about American problems, dwelling on measures by which the United States lags its economic peers.

What might this mythical candidate talk about on the stump? He might vow to turn around the dismal statistics on child poverty, declaring it an outrage that of the 35 most economically advanced countries, the United States ranks 34th, edging out only Romania. He might take on educational achievement, noting that this country comes in only 28th in the percentage of 4-year-olds enrolled in preschool, and at the other end of the scale, 14th in the percentage of 25-to-34-year-olds with a higher education. He might hammer on infant mortality, where the U.S. ranks worse than 48 other countries and territories, or point out that, contrary to fervent popular belief, the U.S. trails most of Europe, Australia and Canada in social mobility.

The candidate might try to stir up his audience by flipping a familiar campaign trope: The U.S. is indeed No. 1, he might declare — in locking its citizens up, with an incarceration rate far higher than that of the likes of Russia, Cuba, Iran or China; in obesity, easily outweighing second-place Mexico and with nearly 10 times the rate of Japan; in energy use per person, with double the consumption of prosperous Germany.

How far would this truth-telling candidate get? Nowhere fast. Such a candidate is, in fact, all but unimaginable in our political culture. Of their serious presidential candidates, and even of their presidents, Americans demand constant reassurance that their country, their achievements and their values are extraordinary.

Candidates and presidents generally oblige them, Barack Obama and Mitt Romney included. It is permissible, in the political major leagues, for candidates to talk about big national problems — but only if they promise solutions in the next sentence: Unemployment is too high, so I will create millions of jobs. It is impermissible to dwell on chronic, painful problems, or on statistics that challenge the notion that the U.S. leads the world — a point made memorably in a tirade by the dyspeptic anchorman played by Jeff Daniels in the HBO drama "The Newsroom."

"People in this country want the president to be a cheerleader, an optimist, the herald of better times ahead," says Robert Dallek, the presidential historian. "It’s almost built into our DNA."

This national characteristic, often labeled American exceptionalism, may inspire some people and politicians to perform heroically, rising to the level of our self-image. But during a presidential campaign, it can be deeply dysfunctional, ensuring that many major issues are barely discussed. Problems that cannot be candidly described and vigorously debated are unlikely to be addressed seriously. In a country where citizens think of themselves as practical problem-solvers and realists, this aversion to bad news is a surprising feature of the democratic process.

"I think there’s more of a tendency now than in the past to avoid discussion of serious problems," says Allan J. Lichtman, a political historian at American University. "It has a pernicious effect on our politics and on governing, because to govern, you need a mandate. And you don’t get a mandate if you don’t say what you’re going to do."

American exceptionalism has recently been championed by conservatives, who accuse Obama of paying the notion insufficient respect. But the self-censorship it produces in politicians is bipartisan, even if it is more pronounced on the left for some issues and the right for others.

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 U.S. politicians, like the skulls in Renaissance paintings that reminded viewers of their mortality.

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.

Indeed, in the current fiscal environment, promising an ambitious effort to reduce poverty or counter global warming might imply big new spending, which is practically and politically anathema. And given the increasing professionalization of politics, any candidate troubled by how the U.S. lags its peers in health or education has plenty of advisers and consultants to warn him never to mention it on the stump.

"Nobody wants to be the one who proposed taking the position that got the candidate in trouble," says Martha Joynt Kumar, a political scientist at Towson University who studies presidential communications.

Of course, the reason talking directly about serious American problems is risky is that most voters don’t like it. Mark Rice, who teaches American studies at St. John Fisher College in Rochester, N.Y., said students often arrived at his classes steeped in the notion that the U.S. excelled at everything. He started a blog, Ranking America, to challenge their assumptions with a wild assortment of country comparisons, some sober (the U.S. is No. 1 in small arms ownership) and others less so (the U.S. is tied for 24th with Nigeria in frequency of sex).

"Sure, we’re No. 1 in gross domestic product and military expenditures," Rice says. "But on a lot of measures of quality of life, the U.S. ranking is far lower. I try to be as accurate as I can and I avoid editorializing. I try to complicate their thinking."

© 2012 The New York Times Company