It must have galled U.S. Education Secretary William Bennett that his moment of truth -- an answer that revealed the real motivation behind his education voucher proposal -- came courtesy of both a Democrat and a Republican.

Bennett was appearing recently before the House Committee on Education and Labor in support of his proposal to convert the current federal remedial education program, which has successfully improved poor children's academic skills, to a voucher system in which some parents of disadvantaged children could "buy" education at a private, public or parochial school.

Rebutting criticism from teachers unions, the national media and education lobbyists that the program would undermine public schools, especially in urban areas, Bennett insisted that tuition vouchers would instead encourage competition and provide poor parents with a choice they now lack.

But during the course of the hearing, under questioning from Reps. Augustus F. Hawkins (D-Calif.) and William F. Goodling (R-Pa.), it became clear that what the Reagan administration really wants is to eliminate the federal role in public education while giving private and parochial schools an opportunity to skim the most promising low-income students and leave the most problematic behind in the public schools.

The two congressmen made the administration show its hand by hammering away at the point that parental choices would be severely limited under a voucher system. Their reasoning was that the average voucher amount, about $600, would not pay enough of the tuition costs to provide low-income families a meaningful choice and that they did not expect many private or parochial schools to accept many of the youngsters.

"The choice would not belong to the parents," protested Hawkins, the committee chairman. "Whether the parents would want to send their children to . . . a private, parochial or other public school would be a choice that was left with the schools, and they can afford to be selective. Most don't want the extra expense and headaches of handicapped or educationally disadvantaged children."

When Bennett denied Hawkins' assertion, the California congressman challenged the Reagan aide to provide a safeguard by writing into the legislation a clause requiring that any school accepting vouchers would have to accept any child who applied, just as public schools are required to do.

Then came the secretary's moment of truth: "If you impose all the requirements [on private and parochial schools] that are currently imposed on [public] schools, you may put them out of business."

Hawkins shot back, "That's unfair competition . . . . Why not put them all on the same basis so that competition will be fair?"

Echoing the same concern, Goodling told Bennett: "As I understand your proposal, you don't bind [private or parochial schools] in any way, shape or form . . . . Why shouldn't the rules be exactly the same if in fact you're talking fair competition? I believe in competition, but it should be fair."

As a former teacher, principal and superintendent, Goodling knew well that public schools have no choice about which students they accept. He recalled that whenever a parochial school principal telephoned him and explained that he had a "wonderful young lady" or "outstanding athlete" who wanted to return to the public school, Goodling said, "I knew I was about to be handed a problem."

The federal programs that Bennett wants to raid to provide tuition vouchers have been steadily losing funds under the Reagan administration. By October, the programs will be serving 3 million fewer young people than when President Reagan took office. That is scandalous enough, but to then tease poor parents into thinking they have a real choice is downright cruel.

Why raid a program that everybody agrees is a success, to pay for one that is unproven? If the secretary wants to prove that vouchers work he should use money from the same discretionary fund he tapped to test merit pay for teachers. That way, at least, he won't make poor children the sword bearers of their own doom.