The Prince George's County Council approved a redistricting plan for itself yesterday, despite the threat of a suit by the county ACLU and the charge by citizen groups that the plan is designed to ensure the survival of incumbents in next year's election.

The ACLU has threatened to file suit against the council on the grounds that the plan will dilute black voting strength on the council. The plan, drawn by a council-picked redistricting commission, will reduce the council from 11 members elected at large to nine single member districts under the requirements of a charter amendment passed last fall.

Currently there are three black members on the council, but under the plan approved yesterday, even supporters concede that blacks would be likely to win only two seats, 22 percent representation in a county that is 37 percent black.

Claire Bigelow, president of the county ACLU, said she was not surprised at the council's decision.

"It was predictable, but it was still a shock when it happened," she said.

"I'm wearing black," said Barbara Brandt of the League of Women Voters. "It's a sad day for the county," she added.

Some south county citizens insist that the plan does nothing for their fast growing area while citizens from Greenbelt were vocally opposed to being placed in the same district with the city of Bowie.

But it was the fact that protecting incumbents was a key consideration of the plan, an admission made by the plan's architects, that drew the most fire from its critics. One council member, William Amonett, said that the redistricting process was tainted last spring when his fellow south county council members lost a fight for greater representation on the redistricting commission.

"I will not be a party to the process any further because it was a bad process," Amonett told the council before abstaining from voting on the three redistricting bills before the council yesterday.

One bill would have adopted the minority plan of the redistricting commission, drawn by south county Republican Ella Ennis. Most of the opponents of the commission plan supported the Ennis plan because it provided three districts south of Pennsylvania Avenue, separated liberal Greenbelt from conservative Bowie and had three seats with majority black populations. But the bill was defeated by a 6-to-4 vote largely because it would have hurt the reelection bids of too many incumbents.

Council member Frank Casula of Laurel said he could not live with it because he would lose voters in south Laurel. Black incumbents Floyd Wilson and Deborah Marshall said their districts under the plan would not provide sufficiently strong black majorities to reelect them, or any black.

The second bill would have adopted a plan drawn under the aegis of state Sen. Tommie Broadwater (D-Landover), but it was rejected by the redistricting commission. Its authors also rejected it in favor of the commission plan, and it received only three votes.

The last bill, approved by a 7-to-3 vote with Amonett abstaining, adopted the commission's majority plan with two precincts shifted to lower the variance in population between the largest and smallest district from 11.2 percent to 7.5 percent and thereby reduce the chance of a court suit over gerrymandering.

The amendment also took incumbent Ann Landry Lombardi out of a district in Bowie she most likely could not win and placed her in a south county district where she would be likely to fare better. But another incumbent, Sue V. Mills, lost her Oxon Hill power base under the plan. She said yesterday she is seriously considering moving south into the new Oxon Hill district, which begins just 1,000 yards south of her home.