A SPECIAL WELFARE PLAN endorsed by many congressmen came under heavy fire Thursday from the agency that would have to carry out the plan. The agency's head told a House committee that the proposed welfare payments could amount to $44 million, twice what the plan's sponsors had guessed. She said that the sort of supervision the congressmen prefer would be inadequate, and that strict enforcement would require nearly doubling the agency's budget and staff. Moreover, she said, the agency would have to shovel out funds so fast that fraud and abuse could not be prevented -- and recouping improper payments later would be very hard. Finally, she testified that key parts of the plan are simply unworkable.

Such testimony might well be fatal to any ordinary welfare bill. But this one is H.R. 1, a quite extraordinary plan for public aid to a select, small group of citizens: those running for the House. Like a very similar Senate bill, the measure is meant to help average incumbents and challengers compete with wealthy candidates and those backed by big-spending interest groups.

Of course, supporters of public financing do not regard it as a welfare plan. But considering it as such -- as an attempt to aid one more group of people who are relatively poor and have trouble surviving in the marketplace -- helps bring into sharp focus the objections raised by Federal Election Commission chairman Joan D. Aikens the other day. For Mrs. Aikens' analysis shows that the drafters of H.R. 1 propose to dish out public aid to politicians willy-nilly, with few of the administrative safeguards Congress demands in connection with food stamps, student loans or any other federal payments you might name.

That's just one side of it. Mrs. Aikens' comments also showed that prudent enforcement and auditing would involve far more record-keeping, reporting and federal inspecting than most politicians are willing to impose on themselves. Then there's the part that most boggled the FEC -- the bill's provisions for extra subsidies to candidates whose opposition spends lavishly. That will work only if every free-spending opponent is willing and able to tell the FEC, up to the minute, when his campaign outlays hit certain heights. And that assumes not just that most politicians are honest, but that almost all of them, in the heat of a campaign, will have impeccable intentions -- and perfect books.

We repeat: In a Congress so touchy about fraud and waste and bureaucratic snarls, a bill that's so unmanageable would not have garnered so many supporters if it benefitted anybody else. Mrs. Aikens' testimony should send the champions of public financing back to the drawing boards. Some other form of public aid for needy candidates may be desirable. This plan is not.