NOBODY EXPECTED the District's convention-center plan to have smooth sailing on Capitol Hill. But since the project has the blessing of Rep. William H. Natcher (D-Ky.) and the full Appropriations Committee, it seemed at least likely to get through the House. Hardly anyone anticipated that it could be stalled, as it was on Friday, by a single representative's objection on a technical point. Yet that is what happened. And because it happened, the fate of the center, along with the rest of the city'scapital budget, has been clouded, at least temporarily. The potential for congressional game-playing and disruptive delays has also increased.

Rep. Robert E. Bauman (R-Md) did have good grounds for his point of order against the capital-borrowing part of the District appropriations bill. As he noted, the law authorizing the city to borrow from the federal treasury expired some time ago and has not been renewed. The obvious question is: Why not? The Senate passed the necessary legislation June 13. As staff writer Jack Eisen has pieced together the tale, the house District Committee was planning to bring the measure to the House floor shortly before the August recess, but the House leadership deferred the bill to accommodate other business. Several days ago the bill was scheduled again, and again set aside. Apparently no one - except Mr. Bauman - realized how the delay could snarl the convention center and the other local projects that require treasury loans.

So far this sounds like one more sad case of general congressional inattention to the city's needs. But wait a minute: It turns out that the borrowing-authority bill was approved by the House District Committee back on April 25. The report was formally filed on May 5. So the real question is why committee chairman Charles C. Diggs Jr. (D-Mich.) waited until late July before trying to bring the bill up on the floor. The District panel has hardly been overwhelmed with other business.Back in May, Mr. Diggs even found time to persuade President Carter to set up that elaborate White House study group on the District's longer-range legislative needs. Yet somehow the chairman failed to see the immediate need for the bonding authority bill. As a result, the measure will now come to the floor - whenever it finally appears - in a much more contentious atmosphere. Mr. Diggs will have to pay attention, belatedly and perhaps exert himself a bit to get the legislation through. We hope that he's prepared to do so. If not, he ought to turn the job over to a colleague who is.