Calling a tax-cutting charter amendment in Montgomery County a "bonanza" for business and a "fraud on the home owners it pretends to benefit," a coalition fo labor, civil rights and education advocates and county employes launched a campaign yesterday to defeat the amendment in the November election.

The formation of the antiamentment coalition yesterday was the first sign of a united effort to defeat the proposal called TRIM, or Tax Relief in Montgomery. Adherents of the measure are already well organized and have lined county streets with red Day-Glo "Trim Taxes" signs.

Eight of the nine county chambers of commerce added their support to the pro-TRIM group yesterday, saying that it is "unfortunately mandaotry" to support the measure to roll back property taxes by a minimum of $26 million because of "complacency, inaction and failure on the part of our present elected officials to 'bite the bullet.'"

TRIM, or Question E on the November ballot, would roll back property taxes from $2.60 to $2.25 for each $100 of assessed value. The loss of revenue from the rollback plus an existing $21 million deficit in this year's budget will require the county to cut at least $47 million from the budget next year to meet expenses, county officials say.

Members of the new anti-TRIM group, called Fair Share Coalition, said county staff budget estimates showed that TRIMs passage would mean a "devastating" reduction in vital services provided by the county government. They cited departmental projections that at least 431 teaching positions would be out, the school crossing guard service would be eliminated, 82 police officers would be laid off, and a minimum of five libraries would have to be closed.

"TRIM's free lunch will become pie in the sky, except for the wealthy," said former County Council president Norman Christeller, who is co-chairing the coalition with former school board chairman Thomas Israel. Neither is currently a county official.

The coalition cited finance department calculation showing that although TRIMs pitch has been to tax overburdened home owners, the top 10 land-holding corporations, including the three major utilities, would be the "big beneficiaries." The companies' combined savings of $2.1 million is a "stark contrast" to the average $55 annual savings per household, said Michael Gildea, coalition spokesman.

Just three weeks before the election the coalitions organization signaled a last-minute effort to counteract the momentum that has been building since the county Taxpayer's League launched its TRIM drive last summer.

The TRIM referendum has become the most heated issue of the November election campaign, with candidates for all offices, including governor and Congress, being pinned down on TRIM in county forums, even though it is a local issue.

"Politicians have received the message that there is a real tax problem," said former school board chairman Israel. "But if the citizens swallow the TRIM meat ax, they will wake up the day after the election terribly sorry for what they have wrought."

But the chambers of commerce which along with the Metropolitan Washington Board of Trade came out in support of the measure, feel differently.

So do the county Republican Central Committee and the Allied Civic Group, a federal of civic associations. "There is no alternative to persuade government officials to reduce the cost of government," said H. Gayle Shaffer, president of the Montgomery County Chamber of Commerce.

Several weeks ago the League of Women Voters was the first group to announce its opposition to TRIM, and the Democratic Central Committee is expected tonight to join the opposition after most of the precinct officials denounced the proposal on Monday.

Some of the opposition to TRIM stems from disagreement over what it would do. The Taxpayers League, the amendment's sponsor, says it would affect only the $2.60 general property tax rate but the county attorney believes that the rollback should apply to all property taxes which include special rates for services such as parks and santiation. This would slice more than $90 million from the county budget.

The dispute, which is likely to result in litigation, will not be resolved before the election.

In addition, TRIM's opponents charge that the amendment would permit a "tyranny of the minority" on the County Council because the proposal would require that six of the seven council members must approve any tax increase in contrast with a simple majority now.

While TRIM's opponents and county government agencies have begun issuing grim lists of programs they say would be cut, the Taxpayers League that started the whole process has issued no assessments of its impact.

"We employ the government to cut out less than essential services," said Karl Schlotterbeck, head of the Taxpayers League. "For us citizens to have to tell them, just makes me laugh."

He regarded the mounting opposition to TRIM with equal casualness. "I have absolute faith in the home owners of this country. Don't forget what we have done," he said, speaking of his successful ballot petition drive, "beginning July 10 with five committee members, 3,000 petitions and a small office with two card tables."

Yesterday, representatives of the 17 member groups, including organized labor, all county employes organizations from firefighters to teachers, the YMCA, NAACP and student councils, predicted the following effects of TRIM:

In the school system, which would have to absorb about $25 million of the minimum $47 million in cuts, 431 teaching positions, 140 clerical and custodial, and 150 administrative jobs would be eliminated. Class sizes would grow. The Head Start program would end and special education programs would be reduced.

At Montgomery College, 99 employes would be laid off to accommodate the $2 million reduction. Tuition increases could result.

Within the police force, which receives one-third of county funds if the huge school budget is excluded, from 82 to 180 officers would be laid off. There are about 740 officers on the force, according to Leonard Simpson of the Fraternal Order of Police.

"We are presently working with the same number of officers we had in 1974, but our calls have increased," he said. "It's a critical level and it is placing every citizen in this county in a precarious situation."

About $1 million would be cut from each of the budget areas of health, criminal justice, social and library services. The Commission on Women would probably disband. Five libraries would close and services could be reduced by one-third at the others. Fourteen of the 80 sheriff's deputies also would be laid off, according to county staff predictions.