The Washington legislature, meeting in special session, today set a primary election for Oct. 11 to select two finalists for the Senate seat vacated by the death of Henry M. Jackson (D).

State senators, under tremendous political pressure, approved a bill creating the special primary election after only 20 minutes of debate. Minutes later, the measure was passed by the House and sent to Gov. John Spellman (R), who signed it immediately.

Without a primary, candidates would face a free-for-all election in which one could be elected with as little as 20 percent of the vote.

Spellman decided to call the special legislative session late Friday, after the state Supreme Court rejected the state Democratic Party's request for a primary. Majority Democratic leaders in the legislature agreed that the bill would be the only measure considered and that it would be passed without changes.

Daniel J. Evans, a Republican who served three consecutive terms as governor, was appointed to Jackson's seat Thursday. The winner of the general election Nov. 8 will serve the remaining five years of Jackson's term. Jackson, a veteran of 42 years in Congress and the patriarch of the Democratic Party here, died of a heart attack Sept. 1.

Two Democrats were emerging as leading contenders for the seat: Seattle Mayor Charles Royer and U.S. Rep. Mike Lowry of Seattle.

Potential Democratic candidates met for an hour here today to discuss party strategy. Among those at the meeting were U.S. Reps. Don Bonker, Al Swift and Norman D. Dicks, Royer and Brock Adams, who served six terms in Congress and was transportation secretary in the Carter administration. Lowry was invited but was in Washington, D.C., and did not attend.

Adams and Dicks took themselves out of the running Friday. Bonker and Swift were reported by aides to be considering the race, but were not researching it as aggressively as Royer and Lowry.

The potential candidates declined comment on the meeting, but party chairman Karen Marchioro said they had discussed polling, campaign strategy and advertising.

A special three-day filing period begins Sept. 14. At the secretary of state's office in Olympia, Evans' son is holding his father's place at the head of the line to file his candidacy. A Lowry supporter is holding his place, second.

In this state's open primary election system, voters are not required to register and vote along party lines, leaving both parties vulnerable to crossover voting. After the primary, the top finalist from each party will be placed on the ballot for the general election.

Four conservative Republicans have announced they will run against Evans: state Rep. Dick Bond, a lawmaker from Spokane; Lloyd Conney, a former television broadcaster; Larry Penberthy, a Seattle businessman, and state Sen. Jack Metcalf, who twice ran unsuccessfully against the late Sen. Warren G. Magnuson.

Due to an editing error, an article yesterday erroneously referred to former Washington senator Warren G. Magnuson as the late senator Magnusion.