THE D.C. COUNCIL has now voted for a tax amnesty -- an idea that is spreading through the country with the speed of chickenpox in a kindergarten. The council, in the budget that it approved this week, is counting on an amnesty to raise $20 million. That's utterly unrealistic. Mayor Marion Barry has warned the council that an amnesty would produce far less -- $8 million to $10 million, in his judgment. Even those figures are pretty optimistic.

An amnesty is simply a grace period in which repentant tax dodgers can pay up without fear of penalties or prosecution. The great example of success is the Massachusetts amnesty of two years ago, which collected $85 million in back taxes. Because Massachusetts has eight or nine times the population of the District of Columbia, and personal income in the same proportio, a comparison points toward Mayor Barry's estimate of the possibilities. But the council needs to be aware of the circumstances in Massachusetts.

Massachusetts, until several years ago, had a reputation for high taxes and slack enforcement -- a bad combination. In the year before it held the amnesty, the state sharply tightened its enforcement both in law and in practice. Tax dodging, traditionally a misdemeanor, became a felony with severe fines and jail sentences attached to it. Criminal prosecutions became visibly more frequent. Previously the state had rarely seized property, but seizures became common -- and well publicized. In one example, the state's collectors swept down the seashore and seized unregistered sailboats. The state now publishes lists of delinquent taxpayers. At the same time it is working to make tax-paying easier by giving citizens more help, redesigning the forms and speeding the refunds. But the amnesty brought in those millions because, essentially, people were given plenty of reason to think that the risks of nonpayment were rising rapidly.

Here in Washington the council seems to be under the impression that it has only to announce an amnesty and wait for the money to roll in. There is no sign here of the enormous increase in enforcement effort that was the key to success in Massachusetts. The amount of money that an amnesty raises is a measure of the inefficiency of the previous collection system, and past performance here in Washington has been good enough that further tightening does not promise any great windfall. To expect $20 million, as the council does, is pure wishful thinking.