To Hendricksen, Hatfield's foe, the Senate race began long ago

Margie Hendriksen's style hasn't changed. The circumstances of her bid for the United States Senate have. More than a year ago the Oregon state senator was eyeing the seat of GOP Sen. Mark Hatfield. The county-fair circuit and the 18-hour days of campaigning are routine for her by now.

It's the news media attention that is new for this Democratic underdog. Her opponent came home this week beset by allegations of influence peddling and a pack of national reporters eager to assess the odds of an upset of one of the most powerful Republicans in Washington.

''I think they just think I was sitting under a tree and an apple dropped in my lap,'' Senator Hendriksen says of the new interest in her. ''But I've been working at this a year.''

No political naif, this divorcee who put herself through law school on a part-time job while raising a daughter says her campaign earned political legitimacy independent of the national focus the Senate race has developed.

She clearly does not want to capitalize on Senator Hatfield's problem, which could hurt her campaign as much as it could help it. An attack on Hatfield's character could backfire within the historic bipartisan coalition behind Hatfield.

''Issues preceded this unsavory tale of Tsakos (Basil Tsakos, the Greek businessman whose association with Hatfield is under investigation), and they will supersede it,'' says Hendriksen.

Those issues, she says, are the key to the three-term senator's vulnerability - a vulnerability that amounted to so little in the eyes of political officials, pollsters, and writers in this state that better-known Democrats wouldn't even consider entering the race. Even with his current problem, Hatfield's reputation is generally perceived here as unshaken.

Hendriksen, a tall Minnesota transplant to Eugene, says Hatfield's ''performance is at total variance with his image.''

Hatfield is viewed as a progressive elder statesman sensitive to peace, civil rights, and environmental issues key to Oregonians. But Hendriksen and her supporters are targeting the senator on those very issues. Their strategy includes these criticisms:

* Hatfield supported confirmation of James Watt who, as Reagan's first Interior secretary, was totally unacceptable to environmentalists.

* The senator did not support the extension of unemployment benefits, even though Oregon has one of the highest jobless rates in the country.

* Since he took the powerful chairmanship of the appropriations committee, Oregon has lost $481 million in home-state projects. In that same chairmanship, he has helped push Reagan budgets that included significant increases in military spending.

* Although he supports a proposed equal rights amendment, he opposes abortion , a position that riles many feminists.

Hendriksen has been somewhat of a marvel to political observers here. ''She specializes in against-the-odds-type campaigns,'' says Diane Luther, president of the Oregon Women's Political Caucus.

A virtual unknown, Hendriksen won a seat in the state House of Representatives in 1980 and built a quick reputation as an ardent feminist and environmentalist. In 1982 she left her perfectly safe House seat in a successful challenge of an incumbent state senator - at the last moment and with little funding.

''Politics isn't for the faint-hearted,'' Hendriksen acknowledges. ''I enjoy the challenge, particularly because people said it couldn't be done.''

The challenge must be sweet for her today, as Oregonians wonder just how far she can go - particularly state Democratic Party officials who six months ago admitted there was no viable contest they could mount against Hatfield.