There are few things in American politics more irrationally ideological, more fanatically faith-based, than the accusation that Republicans are conducting a "war on science."

Does anyone really believe, however, in a science without moral and legal limits? In harvesting organs from prisoners? In systematically getting rid of the disabled?

This last question, alas, does not answer itself. In America, the lives of about nine of 10 children with Down syndrome are ended before birth. In Europe, about 40 percent of unborn children with major congenital disorders are aborted.

All of which highlights a war within liberalism between the idea of unrestricted science and the principle that all men are created equal.

In "Science and the Left," in the latest issue of New Atlantis, Yuval Levin argues that a belief in the power of science is central to the development of liberalism -- based on the assertion that objective facts and rational planning can replace tradition and religious authority in the organization of society. Levin summarizes the liberal promise this way: "The past was rooted in error and prejudice while the future would have at its disposal a new oracle of genuine truth."

But the oracle of science is silent on certain essential topics. "Science, simply put," says Levin, "cannot account for human equality, and does not offer reasons to believe we are all equal. Science measures our material and animal qualities, and it finds them to be patently unequal."

Without a firm, morally grounded belief in equality, liberalism has been led down some dark paths. The old, progressive eugenics of the late 19th and early 20th centuries involved widespread sterilization of the mentally disabled as a form of social hygiene.

Nazism largely discredited the old eugenics. But a new eugenics -- the eugenics of genetic screening and abortion, the eugenics of genetic selection in the process of in vitro fertilization -- is alive and well. Its advocates contend that the new eugenics is superior because it is voluntary instead of compulsory, and unrelated to race. But Levin responds: "Surely the most essential problem with the eugenics movement was not coercion or collectivism. ... The deepest and most significant contention of the progressive eugenicists -- the one that made all the others possible -- was that science had shown the principle of human equality to be unfounded, a view that then allowed them to use the authority of science to undermine our egalitarianism and our regard for the weakest members of our society."

The point here is not to catch liberalism in an inconsistency. At its best, the liberal tradition has combined its belief in science with a firm commitment to the equal value of all -- including the disabled and imperfect.

But science can easily become the power of some over the lives of others. And in their talk of a Republican war on science, liberals may be blinding themselves to a very different kind of modern war in which their own ideals are deeply implicated: a war on equality.

• Michael Gerson is a columnist for the Washington Post Writers Group.