Former governor Daniel J. Evans of Washington, a leader of moderate Republicans nationally during his 12 years in office, yesterday was named to fill the Senate vacancy created by the death last week of Democrat Henry M. Jackson.

Gov. John Spellman (R) announced the appointment at a Seattle news conference. At the conference, Evans said he would be "as vigorous a candidate as I know how to be" in the Nov. 8 special election that Spellman has called to elect a successor to fill out Jackson's term, which runs through 1988.

The appointment of the popular ex-governor caused Democrats to intensify their efforts to alter the terms of the special election, which allows for no primary elections to nominate two candidates and which calls for the winner to be determined by a plurality vote.

The Democratic Party Central Committee has filed suit in the state supreme court asking for primary elections, with a hearing scheduled for Friday.

Evans' appointment gives the Republicans a 55-to-45 majority in the Senate. If Evans holds the seat in the November special election, the Democrats would have to win a net of six seats in 1984 to regain control of the body they lost in 1980.

White House officials say Evans may vote against President Reagan on defense and foreign policy issues more often than did Jackson, but are willing to accept this because they say his appointment gives them a much better chance to hold control of the Senate next year.

Evans, 57, served three terms as governor, and earned a reputation as an honest and progressive chief executive who battled party conservatives and sought reforms in state and federal government.

He was the keynote speaker at the 1968 Republican National Convention, and was considered as a potential vice presidential nominee several times. A student of federalism, Evans served as chairman of the National Governors' Association in 1973-74.

He was first elected governor in 1964, and in the wake of his election began to take control of the state party from the conservatives.

Evans retired from politics in 1977, and after spending a year traveling abroad became president of the Evergreen State College in Olympia.

Evans' appointment to the Senate gives Washington two Republican senators for the first time since 1921 and reunites the former governor with his former attorney general, Slade Gorton, who was elected in 1980.

Republicans said yesterday that they believed Evans was a worthy successor to Jackson, who served 31 years in the Senate and became one of that body's most distinguished leaders. Jackson died Sept. 1 of a ruptured coronary artery at his home in Everett.

GOP officials yesterday appeared confident that Evans could win the November special election, but Democrats argued that Evans, who has not run for office in his state since 1972, may not be as strong politically as many Republicans believe.

"His scores in public opinion terms over the past six years were perfectly ordinary and average," one Democratic analyst said yesterday. "There is no great residual strength and benefit coming out of his years as governor."

At least six Democrats are said to be considering the race against Evans. They are Reps. Norman D. Dicks, Mike Lowry, Al Swift, Don Bonker, former transportation secretary Brock Adams and Seattle Mayor Charles Royer.

But there are efforts under way to persuade Rep. Thomas S. Foley, the House Democratic whip, to enter as a unity candidate.