Prince George's County Council Chairman Gerard McDonough said yesterday that reform of TRIM, the county's tax-limiting charter amendment, is the "primary plank" in his campaign for election to a third term.

Depicting TRIM modification--already backed by about 10,000 people who have signed petitions--as a means of preventing a budget crisis next year similar to the one that necessitated deep school budget cuts this year, Democrat McDonough said: "I think it's the only issue. If it's not resolved correctly, we won't have to worry about any of the other issues."

McDonough supports the "Plus Four" proposal, a new charter amendment that apparently will be placed before county voters in a referendum on Nov. 2. That proposal, backed by PTAs as a way of getting more money for the financially squeezed school system, would raise the ceiling on property tax collections by 4 percent a year.

McDonough said he will push for approval of "Plus Four" despite the political risks of backing a tax increase in an election year.

Backers of the amendment say they have already collected more than the 10,000 signatures needed to place it on the referendum ballot. But, last Tuesday, McDonough has introduced a bill before the council that would place "Plus Four" on the ballot even if the drive for signatures turns out short. McDonough's bill is cosponsored by seven of the 10 other council members and seems assured of passage.

The absolute cap on property tax revenues of $143 million imposed when the voters approved the TRIM amendment in 1978 has been blamed for last month's massive school budget cuts and the layoff of 507 teachers. The County Council of PTAs' drive for signatures to place the "Plus Four" amendment on the ballot, begun in March, had little momentum until the possibility of teacher layoffs was raised.

Council members "were for us months ago," said one PTA official, "but they didn't want to put their signatures on it. They wanted us to prove we could get the signatures. What they were doing is covering their political tails."