THE NEW RESTRICTION on trade with South Africa now making its way through the legislative process represents a growing force in American politics. No longer is opposition to apartheid merely a minority cause or a campus issue or, as Pretoria often claims, a policy whose support is confined pretty much to the administration figures who plug it, naively or cynically as the case may be. On the contrary, it is coming to represent a genuine popular movement commanding its own congressional base.

So it is that the House Banking Committee, hardly a collection of bleeding hearts, has just approved what would be the first congressional curb on economic ties with South Africa. Drafted by Rep. Paul Tsongas (D-Mass.), the amendment would prohibit the Export-Import Bank, a government agency that facilitates American trade, from financing exports to South Africa unless the president determines - and the Congress agrees - that Pretoria is making "significant progress toward the elimination of apartheid."

We are not enamored of the idea of economic sanctions against South Africa. They are a blunt weapon, pressing on both whites responsible for repression and whites seeking reform, on blacks who welcome that pressure as a move against apartheid and on blacks who see it as an invitation to harsher racial confrontation and to hardship for blacks as well as whites. There is also the question, raised in the Senate Banking Committee, of why South Africa alone should be singled out for its internal defects. The administration, we note, did not support this amendment. It objects even to well-meaning congressional restrictions on executive flexibility, and it does not wish to encourage political amendments to economic bills.

The political meaning of the Tsongas amendment is nonetheless clear. The American public's consciousness of apartheid is being raised. From the administration, the idea has spread to Congress that America relations with South Africa will inevitably deteriorate further if Pretoria is not seen to be moving with all deliberate speed to alter the nature of a system that virtually all Americans, regardless of their differences on tactics, regard as an abomination and as a temptation to society-wide violence. Many in South Africa seem to feel their country can tough it out: circumvent or survive the Carter administration and find more "understanding" for apartheid elsewhere in the American body politic. The Tsongas amendment says South Africa is wrong.