A SECOND effort to break a filibuster and bring a South Africa sanctions bill to the Senate floor -- the Reagan administration won the first vote Monday -- is to be made today. Beyond that lies the possibility that new legislation will be offered by those who find fault with the president's executive order. But why? What can come from a continuing effort to deny that Mr. Reagan's adjustments have at least put him in the congressional ballpark?

The likeliest result is a public argument, already stirring, from which some blacks as well as some whites in South Africa will wrongly conclude that there is a reservoir of favor for apartheid in Washington. Projecting that image might be worth the temporary cost if critics thought they could yet obtain stronger sanctions. But after the president's executive order it can only get harder, in a Republican Senate, to gain the two-thirds needed to override a veto. Democrats take on a heavy responsibility by holding out a promise of tougher sanctions than they can reasonably expect to deliver.

Some of those fighting apartheid have strong emotional and tactical reasons to reject anything that looks relatively mild and halfway; the sanctions in the executive order are mild but only somewhat more so than those in the legislation. In this category we put Bishop Desmond Tutu, whose unfair first reaction to the president's action was to suggest that Mr. Reagan is a racist. Of the bishop at least it can be said that he is in the heat of combat on the front line.

For American legislators, different standards must apply. Thanks in good part to them, Mr. Reagan is, however tardily and tentatively, in a new place -- not where they are (or we are), but in a place where he is doing what he long insisted he would never do: accepting and imposing sanctions. The critics should ask themselves what purpose is served by fanning any tendency among South Africans to dismiss the Reagan order as fraudulent and his movement on the issue as negligible. They should consider the advantages of seeing the strength of the president of the United States added to their strength, not subtracted from it.

After all, no set of sanctions currently being considered, congressional or presidential, ever had the chance of forcing white South Africa to its knees. What could be asked -- and this was not insubstantial -- was to send the message that, even in a conservative administration, South Africa was now threatened with losing its single important tie in the world. This is the mark that a consensus American policy can make on the thinking of South Africa's rulers, who need to know that virtually all Americans agree on the imperative of ending apartheid -- not prettying it up, but dismantling it.

Some Democrats, in the name of expressing solidarity with South Africa's blacks, are saying, with Edward Kennedy, that Republicans must decide today "whether they are the party of Lincoln or the party of apartheid." The statement reflects a frightening self-absorption. At this point, we believe, the quest for a stronger bill, or for a position from which to denounce the president, verges on the irresponsible. It is likely not just to be futile, but more importantly to diminish the useful pressure the United States can apply. It can only detract from what is now the opposition's more helpful role -- to hold Mr. Reagan, who has given plenty of cause for doubt about his personal commitment, to his new pledges and to watch what happens as a result in South Africa.