ONCE A MONTH the credit card bill comes in the mail. And once a month some people realize they were a bit too free-wheeling with the plastic wonder. So it goes with the District government's budget. For the last few years -- the last two in particular -- the bills have been coming in way over the city's budget; they have now reached $400 million. Mayor Barry's attitude is that the city will pay later. The trick to doing that, he says, is for the city to go to the bond market to cover as much of the debt as needs to be paid off right away -- about $200 million. When the bill for that loan eventually comes in, the mayor seems to think, the debt won't seem so bad, although with interest payments the District residents will be paying back much more than the $200 million borrowed. The obvious alternative is to cut back on spending and pay off the debts out of the regular budget.

This preference for putting off paying the bill, in the hope that it won't hurt so much later, can be seen right now in the city's handling of the $60 million it overspent this year. The largest single amount of that money -- $23 million -- is owed to the city's pension fund. The mayor admits the debt is real. But on a technicality -- the bill from the federal government came in late -- he says is not going to pay the extra pension debt this year but will allow the bill for pensions in future years to balloon by $23 million. This way the current budget deficit is less painful to resolve; the city can keep living the high life a little longer.

When the bubble bursts, the mayor will find that the city has mortgaged its future. The future we speak of is not 10 or 20 years away, but rather begins Oct. 1, the first day of the 1982 fiscal year. On that day, the city's new budget will already be in the red because there is no provision for the higher pension payments the mayor is pushing into the next fiscal year.

Clearly, something must be done to stop future pension payments from becoming an avalanche that buries the city. Not paying part of this year's pension bill is the exact opoposite of the direction the city should be going in. For one thing, the city needs to do its part to pay off the pension bill if it ever hopes to get a new federal bill to compensate it for some pension excesses that accumulated when the federal government ran the city. But by not addressing the need to put money in the pension fund, Mayor Barry is giving Congress an excuse for not doing its part and is giving the city a false sense of security. One way or another this city is going to get stuck with the bill. The best thing to do now is to eliminate current pension liabilities by cutbacks in programs or by layoffs. ythe city needs to pay its share of the pension costs so it can begin getting put of trouble with pensions for all time.