TRIM IS A BAD idea. But it is likely to be enacted by voters in Prince Geroge's County, and it appears no less than an even bet in Montgomery. There are real dangers in the impact of rapid inflation on the assessment of families' homes, but his kind of legislation is the wrong answer.

TRIM - also known as Son of Proposition 13 - is shorthand for the charter amendments on the Nov. 7 ballot in both Maryland counties that would restrict local taxing powers. The acronym stands for Tax Relief in Montgomery or (in Prince George's) Maryland. The measures are fueled by the exasperation of property owners who see local budgets continue to go up when population is almost static, as in Montgomery, or actually falling, as in Prince George's.

In Montgomery, where the budget has been rising fast, TRIM is widely regarded as a right-wing attack on schools and services. While all the candidates are calling for restraint in spending, most of them have declined to endorse the amendment. In Prince George's the budget, corrected for inflation, is actually a little lower now than it was five years ago. There TRIM travels as a cause of the liberals, protecting working people hard pressed by inflation, and virtually all of the candidates have endorsed it.

The two amendments differ sharply. Montgomery's TRIM would hold the real-estate tax rate to $2.14 per $100 of assessed value. That's a rollback of 35 cents, according to TRIM; some of its opponents argue that it would squeeze out other taxes as well, and have wider effects than its authors intended. If it passes, that will be a question for the courts. The amendments has, at least, a safety valve. In a declared emergency, the county council - by a vote of six out of its seven members - could exceed the ceiling. But even with the ceiling, taxes could continue to rise with a house's assessment.

The Prince George's amendments is much more radical. It would set a ceiling not on the tax rate, but on the tax revenues. It does not attempt a rollback, like its Montgomery counterpart. It would simply declare that the total amount of money raised by county real-estate taxes in any future year could not exceed the present year's revenues - somewhere around $140 million. If assessments rise, the rate will have to drop. There is no provision for inflation. If more houses are built in the county, as they surely will be, the tax on each existing house will have to be cut. There is no emergency clause. The ceiling could be lifted only by another charter amendment and referendum like this one.

The trouble with these arbitrary and rigid limits is that they strip local governments of the discretionary authority that they need to meet unexpected challenges. But if the solution is a bad one, it also has to be said that the distress is real. Sudden increases in real-estate assessments impose unpredictable and inescapable burdens on families with fixed incomes, families with children to educate, families coping with major illnesses. Jumps in assessment increase the segregation of suburban neighborhoods by age and income. It's not the assessor's fault. It's another unhealthy consequence of inflation.

Ideally, the remedy is to shift more of the local tax burden from the property tax to the income tax, which is far better designed to take account of individual families' differing circumstances. But just as tradition gives the property tax to the counties, it gives the income tax to the state. That can be changed, but not rapidly. If the TRIMP amendments pass, they willmean that those two big counties will have less discretion in self-government at home, and will have to turn to the state more often. That would be particularly true in the case of Prince George's, as inflation shrank the value of those fixed local revenues.