WASHINGTON -- Just in time for Independence Day, a conservative think tank has delivered a controversial report questioning whether America's national identity is eroding under the pressure of population diversity and educational slackness.

The threat outlined by the Lynde and Harry Bradley Foundation in its report, "E Pluribus Unum" strikes me as a bit exaggerated. But at a time when Barack Obama and John McCain find themselves debating the "patriotism issue," having a coherent discussion of this matter -- and this short pamphlet is admirably written and well-researched -- is a useful contribution.

The takeoff point for the argument is an observation about the uniqueness of America that was made by Thomas Jefferson -- and by myriad other worthies in the centuries since then. They all have drawn attention to the fact that, unlike other countries, America's national identity rests "not on a common ethnicity, but on a set of ideas."

And so, the Bradley scholars say, "knowing what America stands for is not a genetic inheritance. It must be learned, both by the next generation and by those who come to this country. In this way, a nation founded on an idea is inherently fragile."

The ideas that define this country are found in the Declaration of Independence and the Constitution, as amplified by Supreme Court decisions and statutes in subsequent years. Those ideas have been tested in crisis and in war, and the leaders who steered the nation through those testing times are the heroes whose legacy we celebrate -- Washington, Lincoln, the two Roosevelts.

What disturbs the Bradley scholars is the evidence that our generation is failing to educate the next one on the essentials of the American experiment. "On the 2006 National Assessment of Educational Progress (NAEP) Civics Test," the report notes, "the majority of eighth graders could not explain the purpose of the Declaration of Independence. Only 5 percent of seniors could accurately describe the way presidential power can be checked by Congress and the Supreme Court." The authors also decry the fact that most colleges and universities allow students to graduate without ever taking a comprehensive course in American history and government.

On this point, I think they have plenty of company -- all across the political spectrum. But they have many other criticisms and a variety of suggestions. Some are trivial, such as scrapping Presidents' Day and bringing back Washington's and Lincoln's Birthday holidays. Others are sweeping and controversial, such as telling all colleges and universities to open their campuses to the ROTC.

When it comes to the treatment of immigrants, the Bradley team sees a real threat in such things as multilingual ballots and bilingual classes. Such accommodations to the growing diversity of the population could lead to "many Americas, or even no America at all," they maintain. "Historical ignorance, civic neglect and social fragmentation might achieve what a foreign invader could not."

That degree of pessimism seems unwarranted. The authorities quoted in this report, most of them drawn from conservative academia, manage to overlook the evidence that there is still plenty of vitality in the American system.

Young people may not know the Constitution as well as we would like, but they found their way to the polling places in record numbers this year and joined enthusiastically in many campaigns. And they volunteer for all kinds of good works in their communities.

I have not worried about the fundamental commitment of the American people since 1974. In that year, they were confronted with the stunning evidence that their president had conducted a criminal conspiracy out of the Oval Office. In response, the American people reminded Richard Nixon, the man they had just recently overwhelmingly re-elected to a second term, that in this country, no one, not even the president, is above the law. And they required him to yield his office.

That is not the sign of a nation that has lost its sense of values or forgotten the principles on which this system rests. And that is something worth celebrating on more than the Fourth of July.

David Broder writes for the Washington Post Writers Group. His e-mail address is davidbroder@washpost.com.