Preachers and professors have it hard as presidents. They sermonize too much. Finally the public gets tired of being lectured by those whom they increasingly see as no more upright than themselves. Prophets crumble from feet of clay, and stones shatter glass houses. So it was with Woodrow Wilson and Jimmy Carter, and so it is now with Barack Obama.

The Obama administration is throwing stones at a lot of people — John Boehner, Republicans, tea-partiers, Fox News, Glenn Beck, doctors, insurers, Wall Street, and business in general.

Such invective invites a response, and here the White House is becoming as fragile as glass. We saw that recently in the presidential petulance at supposedly being talked about “like a dog,” and in a touchy press secretary Robert Gibbs unloading at everyone from Rush Limbaugh to Forbes magazine.

Last February, Attorney General Eric Holder, self-appointed racial philosopher as well as the nation’s chief law-enforcement officer, lectured his fellow Americans: “In things racial, we have always been and continue to be, in too many ways, essentially a nation of cowards.” Professor Holder went on to complain that “certain subjects are off limits and that to explore them risks at best embarrassment and at worst the questioning of one’s character.”

Fair enough: Most Americans would be willing to engage Holder in his desired racial seminar — if it were two-sided, and did not devolve into something like the imbroglio over Harvard professor Henry Louis Gates. Before even hearing the facts of that case, remember, the president of the United States, as arbiter of racial relations on campus as well as commander-in-chief, rushed to condemn the Cambridge police for acting “stupidly” and then accused law-enforcement officers in general of racial stereotyping.

In contrast, did Eric Holder’s proposed conversation include questions of welfare dependency, anti-social cultural messages, or lack of personal responsibility — in addition to racism — to explain much higher than average rates of illegitimacy, illiteracy, failure to graduate from high school, and criminal behavior among some minority groups?

So far, Holder himself has never dared to raise such “off-limits” controversial issues. Yet in the case of the Arizona immigration statute, the attorney general was hardly so reluctant. He lambasted the legislation as “unfortunate,” possibly unconstitutional, and leading to racial stereotyping — all before he had even read the law. Cowardly?

Recently, Michelle Obama advised Americans to eat better foods to combat the national epidemic of obesity. She envisions using government power to teach restaurants how to restructure their menus, and helping targeted communities with federal money to improve their collective diets.

Fair enough once more: As a nation we are probably too fat, and First Ladies often seek to better the American condition. But as in the Holder case, does the First Lady, as first professor and preacher, really wish to lecture the American people on their personal sins and to follow that up with federal programs and expenditures? If the issue is to promote better health by using the bully pulpit of the First Family in symbolic fashion, then Michelle Obama might first more quietly start at home with her errant husband.

The presidential role model is secretively a chain smoker — a habit that promotes both heart disease and cancer, and kills millions of Americans each year. At almost every photo op, President Obama is enjoying hot dogs, ice cream, and beer. The president deserves a private life, and his smoking and consumption of fatty foods are his business alone — unless his spouse is suggesting simultaneously that the rest of us must not only avoid such behavior, but seek to fund and institutionalize its antithesis. A voter might well respond to the First Lady’s lectures on diet with something like, “First convince the first husband to stop smoking and to eat better, and then I’ll listen to your advice about my own diet.” Otherwise one might conclude that smoking can keep down weight as effectively as restricting one’s diet can. Such are the wages of a White House of “Do as I say, not as I do.”