On a recent flight here from Sioux Falls, Sen. James Abdnor (R) outlined the accomplishments of his first term in the Senate, admitted that he did not think he had done a good job of telling the voters about them and ruefully concluded as he debarked:

"I guess I'm the only incumbent senator running for reelection this year who has a primary race. How did I get so lucky?"

At his office in the state capitol, Gov. William J. Janklow (R), who announced last night that he is mounting what promises to be a stiff challenge to Abdnor in the GOP primary, had a long, vehement answer to Abdnor's question.

Janklow was just finishing two days that were a politician's dream.

On the first day, his 75-minute presentation of his emergency farm credit program to a joint session of the legislature dominated the local front pages and the evening news. He spent much of the second in the capitol in the pleasant, ceremonial task of honoring people who work with the handicapped.

But for an interview, late in the afternoon, Janklow was mad as hell at Washington and said he wasn't going to take it any longer.

"Every problem we have here is the result of federal government policies or lack of them -- high interest rates, trade, farm foreclosures, water," he said. "Those good old boys in Washington got us into this mess, and they're like a team in last place that never drafts new guys because they like the old guys.

"They don't have the guts to do the right thing -- Abdnor said that and he ought to know, he's one of them. I have a high personal regard for him -- he doesn't lie or cheat, but he doesn't get any hits, either for home runs or for average."

Janklow thinks that determined mavericks are what make things happen -- and so much for Abdnor's plea that he has the seniority and contacts in the Senate to cut deals for the home state.

"I'm not a good old boy and if I become one the people ought to fire me," Janklow said. "No one likes Jesse Helms in the Senate, but they don't go home until they get his work done."

These are the battle lines of what promises to be a close, expensive and divisive primary fight, the only serious one that any senator up for reelection faces this year.

Democrats view Abdnor as highly vulnerable, and capturing his seat is a key part of their plan aimed at regaining control of the Senate. The Democratic candidate will be Rep. Thomas A. Daschle, an early champion of the state's troubled farmers who has won two statewide at-large House races.

Many South Dakota Republicans dread the Abdnor-Janklow fight because they fear that it will tear the party apart and jeopardize gains of the past decade.

Those Republicans had hoped Janklow would run for the at-large House seat and then challenge Sen. Larry Pressler (R), who is up in 1990 and with whom he has feuded for years. If they have to lose one of their senators to Janklow, they would rather it was Pressler.

It comes in a year when Democrats in the state are feeling bullish, partly because former governor Thomas Kneip is running for his old office and is expected to help candidates for the state legislature and other offices, partly because they think such issues as agriculture, trade and defense spending are breaking their way. They also are expected to go all-out for Daschle because he is their most visible colleague, their only statewide officeholder.

The GOP primary will pit two candidates with starkly contrasting styles.

Janklow, 46, is a brash, mercurial outsider who has the nickname "Wild Bill" because of his manner, a combination of prairie populist and street-smart kid from Chicago, where he was born. A former attorney general and legal services attorney on the Rosebud Sioux reservation, he never has had much of an organization or raised much money. He has, however, proved himself a master of obtaining news media coverage.

For instance, on the day Abdnor scheduled a dozen news conferences around the state to announce his candidacy, Janklow outlined his farm credit program. He made the top of the front pages; Abdnor was at the bottom.

Abdnor, 63, is a quiet bachelor farmer, an organization man and party insider who has been lieutenant governor and served 11 years in the state senate and eight years in the U.S. House before being elected to the Senate. He lives on the farm where he was born, constantly encounters former students from his teaching and coaching days and inspires protective affection even among Democrats. At Vermillion, one of his announcement stops, Abdnor recounted that Sen. Rudy Boschwitz (R-Minn.) had counseled him to be more assertive about his record and accomplishments.

"I've got a lot to talk about, but I haven't done a very good job of it and even my friends look surprised when I tell them what I've done," he said. "I didn't realize that you had to say it over and over. Sen. Boschwitz told me that whenever I went through a yellow light I should shout that I'm a U.S. senator, but that doesn't work for me. The cops still pick on me."

Janklow, ineligible for a third term as governor, at first justified his challenge on the grounds that Abdnor could not win reelection, then argued that Abdnor has been ineffective in the Senate.

In response, Abdnor blanketed the state with television spots for three weeks in October and November.

One featured fellow farm-state Republicans, including Senate Majority Leader Robert J. Dole of Kansas, testifying to his effectiveness. Another showed him working nights in his shirt-sleeves while another listed his accomplishments, such as rescuing Rural Electrification Administration low-interest loans and working for a national water projects program.

A poll taken by Richard Wirthlin's organization last November showed Abdnor defeating Janklow 60 percent to 33 percent in a primary matchup and beating Daschle by 51 percent to 46 percent in the general election -- and Daschle defeating Janklow 53 percent to 43 percent.

Most observers think that the matchups of the three candidates are about even, however, and not even Abdnor supporters think his poll numbers will hold up, partly because the poll was taken right after his television blitz.

Abdnor will have the support of the traditional, establishment Republicans, an estimated one-third of the primary voters. He also is expected to get the votes of his fellow ranchers west of the Missouri River who resent the more populous, prosperous eastern half of the state and will have an organization in every county to identify and turn out supporters.

Janklow's appeal is to angry farmers and small-town businessmen, whose turnout in the primary will depend in part on how much he can prod them with anti-Washington rhetoric. In some areas he is considered as a late-comer on farm issues.

"On paper Abdnor wins, but Janklow has the advantage of being on the attack," said one longtime political observer here.

Abdnor talks about the long hours and frustrations of fighting the Agriculture Department bureaucracy and hanging around a House-Senate conference committee trying to save such provisions in the farm bill as funding for emergency feed-grain deliveries in the winter to farmers.

"I had a layover in Denver and spent three hours on the phone with Ag Department people in Washington and Kansas City who were telling me that South Dakota just hadn't gotten its order in," he recalled. "It took all day to straighten it out."

Janklow scoffs at this.

His state is getting a raw deal in federal budget cuts for highways and education, he said. "They get more money for the subway in Washington than we do for water development. It took me three months to work out my farm credit plan because every time I came up with one, they'd pass a new law in Washington.

"Abdnor says he'll keep fighting and winning. Ask him what a loss is."