Ask an average American about the proper relationship between politics and religion and he will likely make reference to the separation of church and state. If he is informed, he may go on to allude to the First Amendment’s language proscribing an established church and upholding the free exercise of religion. But in nearly all cases, a representative American — whether he is Protestant or Catholic, Christian or Jew, religious or secular — will affirm the vital importance of separating religion and politics.

At least in theory. As Hugh Heclo argues in his provocative but ultimately frustrating book, “Christianity and American Democracy” (Harvard University Press, 240 pages, $25.95), the idea of strictly separating politics and religion derives from European political philosophy, which itself developed in reaction to the European experience of mutual church-state interference. In a context of religiously inspired political violence and politically inspired religious oppression, it made sense to assume that the best way to achieve peace and freedom was to separate politics and religion once and for all.

Although Americans have come to use European concepts in thinking about questions of church and state, the two spheres have historically been much more symbiotic in America than elsewhere in the Western world. Reflection on this exceptional American reality prompts Mr. Helco, a professor of public affairs at George Mason University, to propose the image of a double helix to describe the complex interactions between politics (specifically democracy) and religion (specifically Christianity) throughout American history. In Mr. Heclo’s view, democracy and Christianity have developed in tandem, like intertwined strands of DNA, “spiraling through time,” influencing and reinforcing one another. Christianity, in both its Protestant and Catholic forms, has been more democratic in America than anywhere else, just as American democracy has protected myriad groups of Christians from oppression and granted them the freedom to participate fully in the country’s public life.

Until recently, that is. Mr. Heclo’s happy narrative of American history takes a dark turn with “the cultural-political-religious outburst” of the 1960s. It was during this tumultuous decade that “America’s democratic faith and its Christian faith” began to come apart. On one side, secularist ideologues began an effort to purge the public square of religion; on the other, religious believers organized a populist insurrection to take it back. This has been America’s cultural dynamic since the ’60s, and in Mr. Heclo’s view it is likely to continue unless the country’s most devout Christians choose to withdraw from political engagement altogether. Mr. Heclo leaves little doubt that he thinks such a withdrawal would be a disaster for both Christianity and democracy in America.

It is a powerful argument backed up by a broad knowledge of American history. But it is also, in several respects, quite unconvincing. Take Mr. Heclo’s account of American politics, which he treats as synonymous with “democracy.” Because democracy primarily involves using elections to determine the will of a majority of citizens — and because the overwhelming majority of American citizens happen to be Christians of one kind or another — Mr. Heclo believes he is justified in arguing that American politics have always been essentially Christian.

But American politics is about more than majoritarianism. The old civics lesson is right: America is a republic, not a democracy. “Christians” don’t rule the country, and they never have. On the contrary, at any given moment America is ruled by a coalition of groups — many of them incidentally comprised of Christians, but others dominated by Jews and Muslims and secularists of various stripes — that has managed to triumph over other coalitions of groups in an electoral competition for temporary political power. These are the rules of the political game as laid down by the American Constitution. And the Constitution, despite Mr. Heclo’s occasional efforts to assimilate it into his argument, simply isn’t a Christian document. It is a theologically neutral institutional framework for resolving public disputes among citizens with clashing outlooks and interests. (The one attempt in American history to pull the Constitution over to one side of a theological dispute — Prohibition — turned into a farce that one hopes will never be repeated.)

Then there is Mr. Heclo’s assumption — present throughout his study but especially prominent in its pessimistic conclusion — that separating Christianity and democracy is bad for both. The corollary of this view is, of course, that blending them is good. If we’re talking about the efforts of certain radical nineteenthcentury Christians to abolish slavery, or the role of Christian conviction in the Civil Rights Movement of the 1950s and ’60s, then I take his point. But what about the apocalyptic hysteria that swept through large swaths of the nation during the Great Awakening of the 1820s? Or the ignorant fundamentalism that culminated in the Scopes Trial of the 1920s and, in our time, has produced the national embarrassment of the Creation Museum outside of Cincinnati? Or the tendency of evangelical Protestants, both inside and outside of the Bush administration, to evaluate matters of public policy in terms of millennial hopes and expectations?

None of this appears to bother Mr. Heclo in the least. And that should give his readers pause. Although “Christianity and American Democracy” exhibits great learning and obviously grows out of genuine love of America, it is deficient in the one quality that should contribute to any worthwhile study of the relations between politics and religion — political wisdom.

Mr. Linker is a senior writing fellow at the Center for Critical Writing at the University of Pennsylvania and the author of “The Theocons.”