THE STATE DEPARTMENT touts the idea that American policy in southern Africa should be based on consensus, but administration policy is being shredded in political debate.

In the central challenge, the Republican Senate has now followed the Democratic House in approving economic sanctions. Congress did it as much to protest administration attempts to practice quiet persuasion on the South African government as to protest the white minority regime's practice of apartheid. There is much doubt in Congress that sanctions will do other than stiffen whites and inflict further hardship on blacks. But there is broad dislike for what is seen as administration tiptoeing on apartheid. The regime's harsh and continuing reprisals against citizens and neighbors suggest that it is thumbing its nose at Ronald Reagan and at America. This is what Congress is voting against.

Nor are sanctions the end of it. The other day the House followed the Senate in voting to lift the 10-year legislative bar on aiding insurgents in Marxist-ruled Angola. A Congress traumatized by Vietnam, through the Clark amendment, had cut off the covert aid flowing to a favored faction vying to take over from the departing Portuguese. No president since has approved of this congressional shackle on executive discretion. The Reagan State Department has complained that the amendment enacted the Brezhnev Doctrine -- which holds that a communist revolution is irreversible -- into American law. But the main line of policy has assumed that the Clark amendment would stay on the books.

Here again, however, congressional impatience made its mark. Conservatives have long had a fascination with Jonas Savimbi and his UNITA insurgents who are supported by South Africa. More recently some liberals have sought to showthey are tough and anti-communist too. Thus has the administration's policy of encouraging anti-communist resistance movements, in Nicaragua and Afghanistan, caught on in Congress even in respect to places -- Cambodia and now Angola -- where the president has not been out in front.

Some parliamentary hurdles to repeal of the Clark amendment still stand. The administration professes to have no plans to exploit a renewed legal opportunity to aid Savimbi insurgents. But in this moment of passage in southern Africa it is extremely mischievous to revive even the possibility that Washington may get back into regional military intervention. Angola's immediate decision to break off talks with the United States on, among other things, Cuban troop withdrawal, underlies the point.

There is yet another place in which Congress is trampling on the intended subtleties of the Reagan policy of "constructive engagement" with all of the different countries of southern Africa. The House voted limits on development aid to Mozambique, a country the administration has been carefully trying to draw out of the Marxist fold.

The White House objects strongly to intrusions on its policy such as the congressional sanctions votes. It would do better to object even more strongly to what goes on in South Africa. In defending its policy it has too often seemed to be defending apartheid. That is the principal reason why its policy is losing ground in Washington and in southern Africa alike.