Nancy Pelosi, the Democratic leader in the House, sounded like an Old Testament prophet recently when she denounced the Republican budget for its "injustice and immorality" and urged her colleagues to cast their no votes "as an act of worship" during this religious season.

This, apparently, is what the Democrats had in mind when they vowed after President Bush's re-election to reclaim religious voters for their party. In the House, they set up a Democratic Faith Working Group. Senator Harry Reid, the minority leader, created a Web site called Word to the Faithful. And Democratic officials began holding conferences with religious progressives. All of this was with the intention of learning how to link faith with public policy. An event for liberal politicians and advocates at the University of California at Berkeley in July even offered a seminar titled "I Don't Believe in God, but I Know America Needs a Spiritual Left."

A look at the tactics and theology of the religious left, however, suggests that this is exactly what American politics does not need. If Democrats give religious progressives a stronger voice, they'll only replicate the misdeeds of the religious right.

For starters, we'll see more attempts to draw a direct line from the Bible to a political agenda. The Rev. Jim Wallis, a popular adviser to leading Democrats and an organizer of the Berkeley meeting, routinely engages in this kind of Bible-thumping. In his book "God's Politics," Mr. Wallis insists that his faith-based platform transcends partisan categories.

"We affirm God's vision of a good society offered to us by the prophet Isaiah," he writes. Yet Isaiah, an agent of divine judgment living in a theocratic state, conveniently affirms every spending scheme of the Democratic Party. This is no different than the fundamentalist impulse to cite the book of Leviticus to justify laws against homosexuality.

When Christians - liberal or conservative - invoke a biblical theocracy as a handy guide to contemporary politics, they threaten our democratic discourse. Numerous "policy papers" from liberal churches and activist groups employ the same approach: they're awash in scriptural references to justice, poverty and peace, stacked alongside claims about global warming, debt relief and the United Nations Security Council.

Christians are right to argue that the Bible is a priceless source of moral and spiritual insight. But they're wrong to treat it as a substitute for a coherent political philosophy.

There is another worrisome trait shared by religious liberals and many conservatives: the tendency to moralize in the most extreme terms. William Sloane Coffin of the Clergy Leadership Network was typical in his denunciation of the Bush tax cuts: "I think he should remember that it was the devil who tempted Jesus with unparalleled wealth and power."

This trend is at its worst in the misplaced outrage in the war against Islamic terrorism. It's true that in the days after the Sept. 11 attacks, some Christian conservatives shamed themselves by blaming the horror on feminists and gays, who allegedly incited God's wrath. But such nonsense is echoed by liberals like the theologian Stanley Hauerwas of Duke University.

"The price that Americans are going to have to pay for the kind of arrogance that we are operating out of right now is going to be terrible indeed," he said of the United States' response to the Qaeda attacks. "People will exact some very strong judgments against America - and I think we will well deserve it." Professor Hauerwas joins a chorus of left-wing clerics and religious scholars who compare the United States to Imperial Rome and Nazi Germany.

Democrats who want religious values to play a greater role in their party might take a cue from the human-rights agenda of religious conservatives. Evangelicals begin with the Bible's account of the God-given dignity of every person. And they've joined hands with liberal and secular groups to defend the rights of the vulnerable and oppressed, be it through prison programs for offenders and their families, laws against the trafficking of women and children, or an American-brokered peace plan for Sudan. In each case believers have applied their religious ideals with a strong dose of realism and generosity.

A completely secular public square is neither possible nor desirable; democracy needs the moral ballast of religion. But a partisan campaign to enlist the sacred is equally wrongheaded. When people of faith join political debates, they must welcome those democratic virtues that promote the common good: prudence, reason, compromise - and a realization that politics can't usher in the kingdom of heaven.

Mr. Loconte is a research fellow in religion at the Heritage Foundation and editor of "The End of Illusions: Religious Leaders Confront Hitler's Gathering Storm" (Rowman & Littlefield). He served as a member of the Congressional Task Force on the United Nations.