Proposals in congress to vote sanctions against South Africa were lagging until President Reagan imposed sanctions against Nicaragua. As a result of this seeming burden of consistency, the House may approve for the first time, on Thursday, a package of economic restrictions against the practitioners of apartheid. If that happens, it will be a mistake.

The case for sanctions is that white minority rule is at once so odious and so powerful that it must be moved and can only be moved by extraordinary economic pressures applied from the outside. Not to attack apartheid in this fashion, it is asserted, is moral and political appeasement. That the intended beneficiaries may also suffer is set down as a price they are prepared to pay.

But there is a serious, respectable, non-racist case against sanctions. It is that the country's economy is its most effective engine of social transformation, compelling whites to grant blacks precisely the training and education, the livelihood and personal rewards, the choices of where to live and work, the associations and organizations, the sense of their own power and community, that apartheid would deny them. And South Africa's place in the world economy, and especially the high-tech, democratic, politically responsive parts of the world economy, is a prime spur to this process.

All of this is understood perfectly well by the sponsors of the sanction legislation. That is why they have quietly designed the particulars of their bill to make the minimal impact on black jobs and opportunities consistent with sending South Africa a political message. In short, the best thing about the bill is that its effect will be largely symbolic. But that does not make it wise public policy.

The legislation is widely seen, by Democrats, as a rebuke to the Reagan administration's policy of "constructive engagement." That it would be. But it would be a poorly aimed rebuke. The type of engagement that widens blacks' economic advantages and openings is the good kind. What deserves to be criticized in the administration's policy but is not attacked by this bill is the bad kind: the kind that lets too many South Africans ask whether the United States is serious about apartheid, the kind that has American diplomats seem more often to be apologizing for apartheid than demanding its abolition.