FOR A WHILE, there was every reason to believe that the revised plans for a downtown civic center would get the support of Sen. Patrick Leahy (D-Vt.). Months ago Mr. Leahy, chairman of the Senate Apropriations subcommittee on the District, withheld his approval from an earlier plan for a center because it was too costly and it didn't require the business community to share any of the financial burden. City officials were instructed to trim their plan; they were also told to come up with commitments for new private development projects that would generate enough tax money to cover the cost of the center. After several months of work, a modified proposal was duly produced that - in our view - clearly met those requirements. The plan was given a public hearing and then was reviewed and approved by the mayor and City Council. This week, the revised plan was sent to the House and Senate Appropriations subcommittees on the District for their consideration.

Guess what? Sen. Leahy responded quickly to the modified proposal. He said it wasn't good enough. The commitments for private development included in the proposal were not "solid," he said - whatever that means. Carefully nit-picking his way, he went on to complain that city officials had included in their estimates the tax income that would result from developments that would likely occur, regardless of whether a center was constructed. Mr. Leahy said that the city's estimate of $42 million in new tax money each year was an overestimate; as far as he could tell, there were firm commitments for less than $3 million a year in new tax revenue - not enough to cover the $7.5 million estimated as the annual operating cost of the center.

Mr. Leahy may have one point: $42 million a year in new tax money may be an overly enthusiastic projection. But even a very conservative estimate, such as that made by the D.C. auditor, indicates that the District has come up with about $8 million a year in sound commitments. Now, it is true that a letter of commitment is not always an ironclad guarantee that a particular project will go forward - no matter what. But a number of letters from businessmen state explicitly that they plan to build such things as offices, hotels and restaurants only if the center is approved. Many of them cannot say for sure that they will be able to get the necessary financing or insurance, but their letters give assurances that they plan to do so. Still, it appears that Mr. Leahy wants even more of a pledge; he seems to think that the business community must go ahead and start development, no matter what is decided about the center - which is just what he said he didn't like about some of the commitments in the first place.

For all of Sen. Leahy's stated desire to have a center, it seems to us that he is playing games. The city has made a number of changes in the earlier plan, just as Mr. Leahy instructed it to do. The cost and size are considerably less; business participation is high. Eleven of the 49 letters of commitment propose projects that would be built if there were a center; they alone would generate about $13 million in new tax revenue each year, even though the requirement was to come up with only about $7.5 million annually. We think that District officials and supporters of the center have shown good faith - and sound, if enthusiastic, judgment - in doing their work. In the next few days, other members of Congress will look to Sen. Leahy for guidance on this matter; we hope that he, too, will act in good faith.