WHEN THE WORLD BANK appropriation bill finally gets to the House floor, it's going to run into trouble from the chairman of the subcommittee that wrote it. It's an unusual - and ominous - beginning to the debate. The bill was scheduled to come up this week, but the House leaders, fearful of a defeat, have now postponed it until next month. The subcommittee chairman, Rep. Clarence Long (D-Md.), is pressing an amendment to cut the American contribution to the development banks this year by one-fifth, more than half a billion dollars.

He is attempting to deflect even worse cuts, he explains, at the hands of a House that has been stampeded by the California tax vote. Beyond that, he thinks that the United States is funneling too much of its economic aid through the international development banks, over which it has no direct control. The issue of control is now sharpened by the prospect of a World Bank loan to Vietnam.

Last year there was an attempt in the House to prohibit the bank, on grounds of human-rights violations, from lending to Vietnam and several other countries. That was immediately followed by an attempt to prohibit the bank from putting money into any agricultural project the success of which might provide competition to American citrus or sugar producers. Those amendments failed, but they will doubtless be back this year.

The catastrophic unwisdom of trying to impose narrow American interests on this aid should be obvious, when you remember that a rising share of the bank's money is coming from the new rich - for example, the Arab oil states. Their foreign policies, not to mention their ideas about human rights, are quite different from the United States'.

There are large advantages to the United States in running most of its foreign aid through cooperative lending operations like the World Bank. It is run by a board on which both rich and poor countries vote, and there is give-and-take on the loans. Some of the money goes to countries that would not be the United States' first choices. But some of it also goes, at American urging, to countries that other rich donors might not otherwise choose to help.

The World Bank can afford to be rigorous in setting conditions on the use of its money. It doesn't have other political considerations to worry about. Sometimes a U.S. aid mission has to be circumspect, because American military bases, or American economic interests are implicitlypart of the deal. That kind of thing is no concern of the World Bank's and if it doesn't get cooperation the money stops. The same thing is true of the three smaller regional development banks: Inter-American, Asian, African. The U.S. contributions to them, incidentally, are in the same bill as the World Bank money.

But the largest threat to this aid bill is a simple one: the passion for economy that has suddenly seized the House. It is a highly selective passion. It does not extend to dams and highways. It will not reach the Clinch River breeder reactor, or the multibillion-dollar schemes roaring around Congress to help middle-class parents pay college tuition. Instead, the new thrift is focused with dreadful intensity on foreign aid. After all, foreign aid rests on nothing more substantial than conscience and enlightened self-interest, which leaves it in constant jeopardy. The current attitude in the House recalls the story about the family that responded to a sermon on the virtue of thrift by cutting off its contributions to the church and the UGF.