Sen. Daniel J. Evans (R-Wash.), a moderate who entered politics 31 years ago, announced yesterday that he will not seek reelection to a full term in the Senate.

Evans, 62, cited "fundamentally personal" reasons for his decision, but also expressed disappointment about the lack of cooperation in the Senate, and distaste for long and costly campaigning in an era of single-issue politics.

Evans becomes the fifth senator to announce he will not seek reelection next year. On Monday, John C. Stennis (D-Miss.) announced he will retire. Previous announcements had come from William Proxmire (D-Wis.), Robert Stafford (R-Vt.) and Paul S. Trible Jr. (R-Va.).

Evans, who was appointed to the Senate after the death of Henry M. Jackson (D) in September 1983 and was elected in November that year to finish the term, said he would work to elect a Republican president and a Republican replacement for himself in 1988.

Evans made his announcement at a news conference in Seattle following three days of an apparent campaign swing through the state. As late as Monday, Evans had told reporters in Yakima he had not made up his mind. "I've been waiting for that bolt of lightning from above, and I haven't seen it yet," Evans said. He told his staff of his decision only shortly before the news conference. Evans said he reached his decision late Monday.

Evans, who began his political career in 1956 as a state legislator, is the only Washington governor elected to three consecutive terms and has never lost an election in the Democratic-leaning state.

"His decision is a real blow and loss to the state," said Washington Secretary of State Ralph Munro. "He has been the major political leader in the state for more than 30 years. He will be tough to replace."

Those being mentioned as possible successors include every member of the Washington congressional delegation except House Majority Leader Thomas S. Foley.

Bob Chlopak, executive director of the Democratic Senatorial Campaign Committee, said, "We always said this was a good opportunity for a Democratic pickup. This makes it more of an opportunity."

Of the House Democrats -- Reps. Don Bonker, Norman D. Dicks, Mike Lowry and Al Swift -- considering the race, Swift said Monday that he is running regardless of Evans' decision and Bonker said yesterday he is "fairly certain" he will run for the seat.

Speculation on the Republican side includes House members Rod Chandler, John R. Miller and Sid Morrison, who announced yesterday that he was forming an exploratory committee "to measure the potential of my personal candidacy," plus former representative Joel Pritchard; former senator Slade Gorton, who was defeated last year, and former EPA administrator William L. Ruckelshaus.