Prince George's County Executive Parris Glendening, upstaged by two County Council members in the battle over the modification of TRIM, the county's property tax cap, moved yesterday to retake leadership of the issue by convening the first session of a task force investigating alternatives to TRIM.

Glendening told the 16-member committee, which he appointed in mid-August, that he favors essentially the same modification offered by councilmembers Anthony Cicoria and Sue V. Mills. The two councilmembers, who last year campaigned against amending TRIM, proposed last week that the county be allowed to raise the cap on property tax revenue annually by the amount of money generated by new construction. Mills and Cicoria were criticized by some of their colleagues, who said they should have waited for the recommendations of the various citizen groups study modification.

Glendening also told task force members to "feel free to examine all the alternatives, from zero change to a more far-reaching one."

Task force chairman Raymond LaPlaca, a developer and immediate past president of the county's chamber of commerce, said, "I think there are some people on the committee who are not convinced that TRIM should be amended," but that for the majority, "it's not a question of whether, but how."

LaPlaca said the task force also will look at whether the county can provide tax relief for homeowners by changing the assessment process, which has been affected by inflation.

The task force is one of several groups examining alternatives to TRIM, which was approved by voters in 1978 and reaffirmed last fall. Under TRIM, the county can collect only as much property tax revenue as it received in 1979, about $143.9 million. Critics have argued that TRIM allows no growth for inflation or to pay for services needed by new residents and is causing county services to suffer, while TRIM supporters say that the government is run inefficiently and can stand further cuts.

Glendening asked the committee to make its report to him by mid-October and "to work closely with the council to see that our proposal is adopted." Any proposal placed on the ballot by the council would appear before the voters in November, 1984.

He added that Mills, Cicoria and council chairman Frank Casula had agreed to hold off action on their bill until his task force, and one appointed by the council, made their recommendations. Casula and Mills later disputed that, however. Casula said that he will refer the council bill to both task forces for their review and will ask groups to appear before the council. Mills said she will try to move ahead with her bill.

"I see no reason why this bill can't go through," she said.

Glendening also told the task force it should agree on a course of action by the end of the year, so that it could undertake a year-long, privately funded educational effort to convince voters of the need to modify TRIM.