ALMOST UNNOTICED in the conservative tidal wave washing over Washington is the arrival of a newly elected prairie populist straight from the traditions of William Langer and Hubert Humphrey.

Somehow out of that heavy Republican sea, this old opponent of the Vietnam War and friend of Kennedy politics splashed into the House looking remarkably like a liberal although he calls himself a progressive.

North Dakota's Byron Dorgan was the only Democrat in the nation to win an open House seat previously inhabited by a Republican, and he did it in what was widely viewed as a classic showdown between a liberal Democrat and a "New Right" Republican.

This 38-yar-old scourge of corporate America defeated State Sen. Jim Smykowski with 57 percent of the vote in a state that favored Ronald Reagan by more than 2 to 1 and threw all other Democrats up for re-election out of office.

Even more unusually, Dorgan made this leap to Capitol Hill from his post as tax commissioner, demonstrating that he may be the first popular tax collector since his predecessors were driven from the temple.

Of course he wasn't uniformly loved as the state's tax man. Indeed, he used to get a lot of mail addressed to Byron L. Dorgan, SOB.

"Being an optimist and a Norwegian, he said recently. "I preferred to interpret that address to mean, Byron L. Dorgan, State Office Building."

He goes to the stump with whimsy. "It's true I'm smart," he used to tell skeptical voters. "I'm so smart I graduated in the top five of my high school class. . . Nine of us graduated that year in Regent, North Dakota."

"We wrote to a woman to find out why she filed a tax return one year and didn't the next," the former taxman told one crowd. "Her husband wrote back, 'Dorgan, I received your letter. It is with regret I inform you that my loving wife passed away last year and I am taking care of her affairs.

"I want you to know that in the matter of the income tax my wife has gone to heaven. And I hope you go to hell.'"

Instead Dorgan last year headed the North Dakota campaign of Sen. Edward Kennedy because he felt, among other things, that Jimmy Carter "did not deserve reelection." Once Carter was renominated, however, Dorgan supported him against Reagan.

It was his effort to nominate Robert Kennedy in 1968 that led to his first political office. Dorgan had returned to his native North Dakota from Colorado to manage Kennedy's campaign when he caught the attention of Gov. William Guy, who appointed him to fill a vacancy as state tax commissioner.

In 1972 Dorgan acquired some national attention as the only state tax official to write Richard Nixon that the Christmas bombing of North Vietnam made him feel ashamed to be a tax collector.

That year Dorgan ran for the post of tax commissioner -- it is the only job in the country. He prevailed with 70 percent of the vote and settled down to his crusade against tax-evading corporate America.

He had discovered that many large corporations earning income in North Dakota never filed returns. Moreover, others were paying less than they should by making it appear they made most of their money in low-tax states.

"It was easy to send an auditor to the drug store or the gas station or the mom and pop store in Fargo or Grand Forls," Dorgan said. "But North Dakota never bothered to send them to big corporate headquarters of firms that were competing against them."

"I don't mean to assert that all corporations are crooks," he said. "But there are some that are very derelict. Some flagrantly violate all principles of accounting and walk the thin line betwen fraud and unethical practices."

So he started sending out auditors: "We'd send a guy to New Jersey for three weeks and he comes back with a check for $625,000. It was phenomenal, incredible. We would start checking on these people and they would just pay up."

Not all were that easy. One of his bigger coups was Western Electric. This AT&T subsidiary was the prime contractor on the ABM development near Grand Forks and was pretending to be some nontaxpaying extension of the Department of Defense.

"We couldn't get their attention," Dorgan said. "So we sent them a bill for $54 million, which under North Dakota law is fixed unless protested within 30 days. Well, that got their attention. Within weeks they were there with eight lawyers and a general.

"The first thing they tried to do was say, 'Hey, you're crazy. Nobody has ever tried to tax a missile system.' Then we went to court. Three years later North Dakota got [in a settlement] over $4 million."

This was part of the some $23 million he collected in unpaid corporate taxes and helped develop his potent political theme: "Little people slave and pay taxes while big corporations duck everything possible."

He also went after individual tax evaders. At one press conference he declared he would prosecute 250 unnamed "hard core" evaders with incomes over $10,000 a year. Within two weeks 280 people who had never filed somehow got around to it.

"It's funny. People started calling me at home at night apologizing. They would say they had just forgot to send in their returns but they would be getting them right in in the morning."

Meantime, he ran for the House in 1974 against former Rep. Mark Andrews, who was elected to the Senate last year. Andrews, who usually won with about 70 percent, was held to 55. "I singled his tail a little bit," Dorgan said. In 1976 he was reelected tax commissioner, with almost 80 percent of the vote.

Last year, when that House seat opened, he was ready again. He promised to vote for a strong America but talked also of the need for meaningful arms limitations talks and was anxious about the proliferation of nuclear weapons.

While he spoke of cutting waste from the federal government, he also called for increasing medical benefits for the elderly and the poor.

"I consider myself a progressive politician. I want to speak up for the little person who otherwise doesn't seem to have a voice in government. But I don't like liberal/conservative labels.

"I regard myself as liberal on some issues and conservative on others. The progressive labels fits pretty well.

"People think it was a conservative year. But I think you can go out and be a voice for the folks who don't own big blocs of Exxon. You can be a voice for the people buying gas at the pumps who are angry that it is helping Mobil buy a department store.

"I think you can be a voice for these people."