Among the most durable myths of American public life is that conservatives are more authentic in their religious faith than liberals and progressives. Certainly this arrogant presumption prevails on the religious right, where commentators and politicians routinely denigrate the sincerity of Christians such as Hillary Rodham Clinton and Barack Obama, whose irredeemable sin is that they also happen to be Democrats and candidates for president.

With characteristic condescension, an editor of the right-wing Weekly Standard dismissed public expressions of faith from the left as both pointless and worthless. He declared that the Democrats can only attract Christians who are “religious in the way that Hillary Clinton is religious, which is to say a very liberal Protestant sort of view, in which they believe in everything but God.” That quip is quite mild compared with typical fiery denunciations from the religious right, branding the Clintons as instruments of Satan and Obama as an Islamist in disguise.

Both senators are fully capable of expressing their personal beliefs and defending their theological views whenever that seems appropriate. Like other presidential candidates on both sides of the partisan aisle, they can offer substantial evidence of their religiosity to churchgoing voters. Of course, that won’t discourage conservative critics from noisily denigrating them (and all Democrats) as apostates and heretics.

In these endless squabbles over the faith of politicians, however, it is striking how rarely right-wing officials and preachers find themselves on the defensive about their faith — no matter how hypocritical and even hateful their behavior may be. These supposed men of God may be adulterers; they may be crooks and liars; they may cultivate the wealthy and corrupt while despising the poor and lame; yet somehow, their adherence to Scripture isn’t subject to doubt.

It is time to stop being so damned polite — and to start asking them a few hard questions.

The lucrative business known as “Christian broadcasting,” for instance, is also the source of the most fervent, widely heard attacks on the sinfulness and apostasy of liberals, progressives and Democrats. Now may be the time to ask: What exactly is so “Christian” about these multimillionaire Republican televangelists, who plainly look upon their flocks as so many fat sheep to be fleeced?

From the preeminent Pat Robertson of Christian Broadcasting Network to Paul Crouch of Trinity Broadcasting Network to lesser but rising figures such as Rod Parsley and John Hagee, these mountebanks persuade their hopeful viewers to send in checks, frequently promising that the Lord will bring prosperity to the poor and restore health to the ill if only those “tithes” are sufficiently generous. When the expected miracles fail to materialize, say the preachers, it is only because the faithful didn’t believe strongly enough — or didn’t donate enough money. Contributions are routinely misused to purchase luxury estates, private jets and lavish vacations in Vegas, rather than to propagate the Gospel in Asia or to relieve suffering in Africa, as advertised.

Meanwhile, these fundamentalist shepherds promote hostility toward homosexuals, liberals, feminists and Muslims and toward Christians who don’t share their dogmas, all in the name of the Prince of Peace. They steer their followers toward right-wing ideology and Republican candidates, skirting or ignoring the laws that prohibit churches from engaging in partisan politics. They abhor Social Security, Medicare, national health insurance and every kind of government assistance to families, even while they eagerly cash in on “faith-based” federal handouts.

Now presumably the faith healers and prosperity preachers can cite biblical verses to justify their dubious behavior, but it is hard to imagine that any of this is what Jesus would really do. Too many times in recent years the blustering enforcers of family values and public piety have turned out to be frauds. The unhappy examples range from Ralph Reed’s gambling connections and Newt Gingrich’s infidelities to Ted Haggard’s secret gay lifestyle. The latest episode involves Sen. David Vitter (R-La.), another conservative Christian moralist, whose name just turned up in the phone book of the “D.C. Madam.” (He once said that Bill Clinton deserved to be removed from office for his sexual infidelity, but now shows no sign of resigning his own seat.)

When these Christian gentlemen are caught with their pants down or their hands in the cookie jar, they invariably claim that they have repented and prayed for forgiveness. In a spirit of generosity that they often lack, let us hope that they can cleanse themselves of wrongdoing. But let us remain skeptical whenever an ostentatiously pious conservative presumes to judge the faith of liberal Christians — including candidates for president.

Joe Conason writes for The New York Observer.