FORTUNE — Once upon a time our leaders were honest and brave, and the Republic flourished. But nowadays the virtues of the founding fathers have given way to corruption. The Republic is experiencing political and economic turmoil. Some even argue that it has entered terminal decline and will soon be supplanted by upstart rivals from the East. If they wish to avoid this dark fate, the citizens of the Republic must rediscover their ancient virtue.

Jeffrey Sachs didn’t invent this line of argument — credit for that goes to Marcus Tullius Cicero (106-43 BC), the grumpy statesman and orator who witnessed the last days of the Roman Republic and its transition to an imperium under Julius Caesar. But Sachs reprises it to good effect in his latest book, The Price of Civilization. Sachs began his career as a globetrotting macroeconomist who helped Russia and several of its former satellites manage their variously bumpy transitions from communism to the market system. He later shifted his attention to poverty in the developing world, publishing bestsellers like The End of Poverty, in which he argued that extreme poverty could be eradicated in our lifetimes by substantial doses of foreign aid directed to medical, agricultural, and educational reform, along with microcredit and other financial innovations. Nowadays he runs the Earth Institute at Columbia University, where he teaches courses on sustainable development and healthcare policy. And he still travels the world, delivering advice about globalization and development to everyone from Bono to U.N. Secretary General Ban-Ki Moon.

In his new book, Sachs turns his agile mind on the U.S. and finds a big fat mess — literally. In Sachs’ telling, Americans have grown obese, ignorant, and apathetic. Our schools are collapsing, our infrastructure is crumbling, our great companies are selfish and predatory. Our politicians have been thoroughly corrupted by power and money and live mainly to serve the interests of their wealthy patrons. Plus, we watch way too much TV. “America has developed the world’s most competitive market society but has squandered its civic virtue along the way,” he writes. “Without restoring an ethos of social responsibility, there can be no meaningful and sustained economic recovery.”

While Joe and Jane Citizen take their share of blame for America’s ills, Sachs focuses most of his ire on our leaders and on the super-wealthy corporations and plutocrats who allegedly pull their strings. It’s a bit odd to see a pointy-headed, Davos-frequenting technocrat like Sachs finding common cause with the Tea Party on one hand and the Wall Street protesters on the other, but there it is: This card-carrying member of the international elite clearly shares the anti-elitist anger that has become so prominent a feature of our public discourse in recent years.

Sachs piles up a mountain of evidence, much of it familiar, in support of his case. He notes the dramatic wealth disparities of U.S. society, and traces them to the 30-year trend of anti-tax, pro-market fervor that started with Ronald Reagan’s famous declaration that government was the problem, not the solution to our problems. He also cites demographic trends: In recent decades Americans have increasingly sorted themselves by class and ethnicity, which is fine if you live in a wealthy suburb with good schools, but terrible for the future prospects of anyone born into our rapidly growing underclass. This in turn bodes ill for American economic competitiveness, as we create entire generations of young people who lack the skills needed to succeed in a viciously competitive, increasingly globalized economy.

Elsewhere in his fast-paced jeremiad, Sachs deplores the cozy revolving-door Washington culture where politicians so often become lobbyists after they leave office, usually representing the wealthy corporate interests that funded their campaigns. And it’s partly our fault that the foxes are running the henhouse, because Americans have grown dangerously disengaged from civic life. He cites the sociologist Robert Putnam’s influential 2000 book Bowling Alone to evoke a society of alienated suburbanites who watch TV by themselves while getting fleeced by modern malefactors of great wealth.

While Sachs does call for higher taxes on the wealthy, he’s hardly a doctrinaire big-government liberal. During his work in Eastern Europe he was often criticized for putting too much faith in free markets, an attitude that arguably helped nurture the kleptocratic crony capitalism of post-Soviet Russia. In this book he focuses more on the limits of market capitalism, invoking Paul Samuelson’s vision of efficient markets complemented by an activist government. He writes: “[T]he market by itself is not equipped to achieve the triple bottom line of efficiency, fairness and sustainability. The market system must be complemented with government institutions that accomplish three things: provide public goods such as infrastructure, scientific research and market regulation; ensure the basic fairness of income distribution and long-term help for the poor to escape from poverty; and promote sustainability of the earth’s fragile resources for the benefit of future generations.”

Some of Sachs’ riffs are more convincing than others. He makes the familiar argument that the corporate news media have worsened our civic plight in their restless search for ratings by promoting noisy extremists on the right and left while ignoring the quieter, more moderate center. But he ignores the fact that the corporate media have been steadily losing mindshare and revenue to the Internet, where anybody with a keyboard and a broadband connection can broadcast news and analysis to the world. The ideological extremism and intolerance that Sachs decries are flourishing in the egalitarian, largely non-corporate blogosphere, which often makes Fox News and MSNBC look like Walter Cronkite’s CBS. In this instance at least, it’s not at all clear that cynical corporate plutocrats are really driving the bus as Sachs would have us believe.

At times Sachs contradicts himself. Early in the book he compares the dramatic income inequality of modern America to the “gaudy excesses” of the Gilded Age. Later he cites Max Weber’s The Protestant Ethic and the Spirit of Capitalism as evidence that early modern capitalists were better than their selfish, greedy modern counterparts because they believed in deferring immediate gratification, saving for a rainy day, and using their wealth to help the less fortunate. Which is it, professor? There’s truth to both perspectives, of course: the gilded age was certainly a time of wretched excess, yet robber barons like Andrew Carnegie and John D. Rockefeller also enriched American society through wise philanthropy at massive scale. But Sachs makes no effort to resolve the contradiction, simply evoking two alternate realities to support different stages of his argument.

These are quibbles, however. At the end of the day, Sachs has produced an eloquent call for American civic renewal based on moderation, compassion, and cooperation across the lines of class, ethnicity, and ideology. While no reasonable person would disagree, it’s less clear how this great renewal is supposed to come about. The historical record is not especially encouraging on this point. After all, few Romans heeded Cicero’s call for a return to ancient Republican virtue. Cicero himself was ultimately put to death at the orders of Mark Antony, who apparently resented his attacks on the autocratic Caesarian faction in Roman politics. Sachs, on the other hand, will probably sell a lot of books and spend even more time on the Charlie Rose show, preaching to other smart, well-intentioned centrists. The rest of the country would do well to tune in.