ONE QUESTION kept occurring to us during the prolonged conflict in New York City over who would be its next mayor: Why would anyone want to be mayor of New York right now anyway? And, sure enough, not long after the voters had chosen Edward I. Koch on Tuesday, the next step took place in the continuing saga of the city's financial crisis. A vote of little confidence by one of the nation's credit-rating firms doomed the city's plan to borrow money on the open market. This act in itself does no immediate harm to the city. But it underlines the task the new mayor faces in trying to stave off financial collapse.

We don't pretend to understand all of the ins and outs of this particular decision by Moody's Investor Service. Indeed, its statement on the safety of the short-term notes in question and the low rating it gave them seem somewhat inconsistent. It said it could "foresee nothing which should prevent the timely payment of these obligations" but went on to rate them low apparently because of the chance that the city might go bankrupt sometime in the future. That sounds to us more like a predictio of the future than an evaluation of the present.

We do understand, however, some of the implications of this event. It means that the state of New York and the U.S. government will have to continue to be the bankers for New York City for some time to come. And it means that Mayor-elect Koch will face an extremely difficult task in getting the city's revenues and expenditures into a position that will pass muster with those whose views on balance sheets dominate the money market's opinion of borrowers. The city had created all kinds of extraordinary protective devices to ensure the safety of this issue of short-term notes, and the disdain with which these were treated suggests it will be years before the city can borrow money in normal channels.

Congress is hardly likely to be pleased with this situation. This $2.3 billion federal loan program expires next June, and several congressional leaders have expressed the desire that it not be renewed. This recent development comes close to requiring either Congress to renew the program or face the immediate collapse of New York City's finances. We do not envy Mr. Koch the task of explaining the whys and wherefores of all this to his former colleagues on Capitol Hill. But, then, he knew that was a possibility when he started running for mayor.