SO THE ADMINISTRATION'S "white paper" on foreign communist backing of the El Salvador in surgency is defective. The evidence of Soviet, Cuban and other support that was proclaimed to be "definitive" in February turns out, upon closer inspection by this newspaper and The Wall Street Journal, to be arguable and, in some cases, plain wrong. We had found the documentation substantial in the first go-around. Upon reflection we're not surprised to see questions being raised 100 days later. Politics is not arithmetic: everything is arguable, especially in El Salvador-type situations, and it seems almost an iron-clad condition of American political life that these foreign policy white papers end up being in some degree discredited in a few months' time. But the fact is that the nature of the foreign role in El Salvador never could be established by documents. That's not how these things work. The administration overreached in resting its case on a hastily prepared brief that others could dissect at leisure.

Some of the administration's critics, making a point the Post and Journal news articles on the white paper disavowed, suggest that official policy has been undermined. They see deceptive means being used to sell a flawed policy. But we don't see deception -- simply error, and not necessairly grievous error: to the new questioning of its white paper the State Department has some interesting and vigorous rejoinders. Concern over a foreign role in El Salvador was not, after all, a Reagan invention. The Carter administration had become exercised enough about it to restart military aid, and even Robert White, the Carter ambassador fired for challenging Reagan policy, has accepted that there is a certain foreign role.

As for the policy, from the start it has been essential to respond to the foreign component, but not to overrespond. The risk in the white paper was never so much that it would take in the public as that it would take in the administration -- reinforce its inclination to treat El Salvador principally as an arena in which to do military battle against international communism, neglecting the struggle's domestic roots. The white paper was not especially important, we suspect, in bringing Congress and foreign friends to their current levels of support for the Reagan approach. The administration was going to get most of that support anyway. It was probably more important in shifting the balance inside the administration toward the lopsided military emphasis that still characterizes American policy.