OREGON has long been known as ''a laboratory of reform,'' as the Almanac of American Politics puts it. The first state to institute ballot initiative, referendum, and recall, it has also led on such politically touchy issues as statewide land-use planning and the legalization of physician-assisted suicide. Now, with the need to replace disgraced United States Sen. Bob Packwood (R), Oregon is pressing toward yet another first: nominating and electing a member of Congress by mail-in ballot. In response to the unanimous recommendation of the Senate Ethics Committee that the five-term lawmaker be expelled for sexual and official misconduct, Mr. Packwood has agreed to resign Oct. 1. Gov. John Kitzhaber (D) announced last week that a special party primary will be held Dec. 5. A special election to fill Packwood's slot will follow on Jan. 30. ''We have a unique opportunity with this special election to reaffirm one of democracy's most basic principles: That government is best which is governed by the most,'' says Oregon Secretary of State Phil Keisling. The state has been holding mail-in elections on local issues since 1981, and the turnout in such cases typically is higher than those where voters go to the polls. Oregonians are known as independent-minded voters, historically choosing a mix of Republicans and Democrats for state and national office. But the timing of the elections, the fact that they will be by mail, and the lineup of declared candidates all seem to benefit Democrats - at least at the beginning of the race. ''The election to replace Packwood is likely to be a high-profile election with a high-voter turnout ... favoring Democratic or independent candidates,'' observes longtime political commentator Russell Sadler. At the same time, Oregonians (who line up 43 percent Democrat, 37 percent Republican, and 21 percent independent or minor-party members) have been electing only Republican US senators since 1968, when Packwood ousted four-term veteran Wayne Morse (D). All of this should make for a lively campaign, including what observers say is likely to be a strong showing by female candidates from both parties. All three Democratic members of Oregon's congressional delegation - for whom the race is something of a free shot at the upper chamber since they keep their House seat if they lose - have jumped into the race. THE most liberal is Rep. Elizabeth Furse. She has the smallest campaign war chest, but is expected to receive financial support from ''Emily's List,'' a group of donors who help mostly Democratic women, and the Women's Campaign Fund, a political action committee that finances women of both parties. Rep. Ron Wyden has more than $500,000 in his campaign fund, one of the highest in the whole House after the 1994 election. He is known as ''one of his party's most creative legislators,'' according to the Almanac of American Politics, with ''a knack for making the counter-intuitive political alliances which are so helpful in passing unfamiliar measures through the House.'' Rep. Peter DeFazio's strength has been in taking an independent stand on issues. And the fact that he has strongly opposed the Clinton administration in some cases, such as the North American Free Trade Agreement, should work in his favor at a time when bucking the Washington establishment is as popular here as it is across the country. Among GOP candidates for the Senate seat is state education Superintendent Norma Paulus, who is well-known and highly regarded around Oregon as a popular pro-choice moderate. Like Furse, she is likely to receive financial help from national groups interested in increasing the number of women in Congress. Another strong Republican candidate is state Senate President Gordon Smith, a young and striking politician who also benefits in terms of campaign resources from a large personal fortune acquired as owner of a frozen-food company. In recent years, the Republican Party apparatus here has come under the influence of the Christian right. Whether this features in the upcoming elections remains to be seen. ''The tragedy of Packwood's fall from grace,'' says political commentator Sadler, ''is the declining influence of the Oregon Republican Party he was so successful in building over the last three decades.''