Two Republican House members have announced that they will not seek reelection, for different reasons.

Rep. Robert McClory, stung by a Supreme Court decision that "undermined" his Illinois district, announced yesterday he will not run for reelection, ending 20 years in Congress and his chances to chair the important House Judiciary Committee.

Rep. Millicent Fenwick of New Jersey says she has decided to run for the Senate seat held for 23 years by Democrat Harrison A. Williams Jr., who has been convicted in an Abscam case.

Fenwick, who turns 72 next month, said in a letter mailed to people in New Jersey that she has decided to seek the seat and feels she can be a "strong, effective member of the Senate."

McClory, who will be 74 this month, gained national prominence as a member of the committee when it considered impeaching President Nixon. He is the first casualty of the redistricting required by the 1980 census.

He said he would retire to avoid a costly and divisive primary fight against Rep. John E. Porter (R-Ill.), whose suburban Chicago district was moved westward into part of McClory's home turf.

McClory said he changed his mind about fighting Porter Monday night after first declaring he would make the race.

The Supreme Court on Monday declined to reverse an Illinois federal court ruling upholding a remapping that would pit several incumbent Republicans against one another.