FOR THOSE INTERESTED in the plans for a downtown convention center, there are two things to watch right now: the action of Sen. Patrick Leahy (D-Vt.) and the calendar. Sen. Leahy is chairman of the Senate Appropriations subcommittee that oversees the District's spending, and it is his responsibility to see to it that action is taken on the center, before Congress's proposed October adjournment, rather than to let plans die this year.

We would like to assume that Sen. Leahy will carry out this responsibility. But his recent record on this issue gives us pause. It was he who withheld his approval from an earlier plan for a center because it was too costly and didn't require the business community to share any of the financial burden. And for that he deserves credit, since that action forced city officials to trim their plan and to produce commitments for new private development that would produce tax money to help cover the cost of the center. But after all that had been achieved, Sen. Leahy said no, it still wasn't good enough, the commitments were not "solid," and estimates of tax income were too high.

That kind of game-playing could go on forever - which is why Sen. Leahy's moves in the immediate days ahead bear careful watching. As it stands, his subcommitee plans to hold hearings on two mornings - Oct. 10 and 14, just before the scheduled congressional adjournment. Those late dates may have been unavoidable, but they should not be used as an excuse for pigeonholing the center plan along with some flimsy excuse about how the subcommittee needs more information from the business community, say, or a report from the General Accounting Office. That kind of information should be requested immediately.

And rather than reflexively following their chairman on this matter, members of his subcommitee should take a serious look at the revised convention center proposal now so that they can reach their own conclusions. In doing so, they may wish to consider, too, that the plan has met with the approval of their counterparts in the House ana has been approved by the elected mayor and City Council. As for concern about support in the general community, it is significant that even in wards from which the most vigorous objections have been voiced, the City Council members who have supported the center plan have been reelected. So it is now up to Sen. Leahy and his colleagues to respond in good faith - rather than to let the clock run out.