Even his friends concede that Sen. George McGovern (D-S.D.) may command the support of a majority of his constituents for only one day every six years -- just long enough for this conservative prairie state to reelect one of the Senate's leading liberals.

McGovern -- targeted for political extinction by New Right political action groups and opposed by a popular three-term Republican member of the House -- has good cause to hope the axiom is at least partly true.

For, even by his reckoning, if the doesn't do better against Rep. James Abdnor (R-S.D.) in November than he's doing now, he will go down to defeat. His seat would be the biggest trophy yet for the ultra-conservative political movement that is using independent, largely negative campaigns to try to rid the Senate of its most liberal members.

Led by the National Conservative Political Action Committee (NCPAC), a whole host of antiliberal, antiabortion antiabortion and antigun-control groups have zeroed in on McGovern and Sens. Frank Church (D-idaho), Alan Cranston (D-Calif.), John C. Culver (D-Iowa) and Birch Bayh Bayh (D-Ind.), withMcGovern as their prime target.

Uninhibited by formal ties to the Republican challengers or by federal spending limits, NCPAC has mounted a free-wheeling, million dollar drumbeat media campaign to fire up opposition to the incumbents before their challengers come along with lower-key, standard GOP sales pitches. "Exposing radicals" is what NCPAC's director, John Dolan, calls it. "Poisoning the well," McGovern calls it.

His voice barely rising above the engine's hum as the car sped through gently rolling cornfields near where he was raised as a Methodist minister's son in Mitchell, S.D., McGovern matter-of-factly conceded recently that he is badly trailing Abdor right now.

The gap is probably 10 percentage points, he said, twice the lag he found when he polled South Dakota at the start of his campaign for a fourth Senate term last winter.

"It wouldn't be the first time that I was that far behind and won," he said, insisting that he wasn't demoralized. "They always overdo the attack, and I always work pretty hard in a campaign."

His voice rose ever so slightly. "He [Abdnor] won't win without a helluva fight. I haven't been in politics this long to peter out now."

McGovern, his party's presidential nominee in 1972, has been accused of being everything from a political "baby-killer" to a buddy of Fidel Castro in a year-long barrage by the New Right groups, and the normally amiable, low-key senator is angry.

He had seriously considered not running for a fourth term, but the anti-McGovern groups got his back up, especially with literature questioning his loyalty and bearing a caricature of McGovern with a bulls-eye target over his heart.

"We've had enough of that kind of thing," he said. "I got mad. After all this time, why should I turn everything over to those people? I said to Eleanor [his wife], 'Dammit, I just feel like staying in long enough to keep them out.'"

Always an aggressive campaigner, he has been campaigning even harder this year, dashing back and forth between Washington and South Dakota almost every weekend, riding in small-town parades and, yes, even speaking to Veterans of Foreign Wars conventions.

He talks a lot about defense these days, candidly admitting he probably wouldn't be spending so much time on the subject if he weren't running for reelection, but insisting that he hasn't changed any of his positions because of it.

Although he has opposed the B1 bomber and maintains that President Carter was right to cancel it in 1977, McGovern now advocates building some type of new bomber, saying he could support, as a second choice, one that has been dubbed "son of B1."

While he does not duck the liberal label, as some nervous liberals have done, he also takes pains to assert his adherence to what he calls the conservative values of family life and resource conservation. He decries big government in a populist sort of way, saying, "I don't like big government, but I don't want to see its place takenby giant corporations who don't care about people."

He talks even more about "South Dakota's investment in George McGovern" -- nearly a quarter-century in Congress including three terms in the Senate, where the 57-year-old Senator now has more seniority than the combined total of the rest of the state's four-member congressional delegation. c

"When the big 1981 farm bill comes before the Congress next year, which would be better for South Dakota -- a senator who is next in line to be chairman of the Senate Agriculture Committee . . . or a freshman Republican who may, or may not, even be assigned to the Agriculture Committee?" he asks. w

Republicans don't know whether to laugh or cry when he invokes no higher authority to prove his point than the late Sen. Karl Mundt (R-S.D.), who successfully used the seniority argument against McGovern in 1960.

It's a side of George McGovern, the quintessential espouser of liberal causes at home and abroad, that Washington doesn't always take the time to see. But it's basically why he's there. He's a masterful wheat-cows-and-water projects politician, a prairie populist with passions that extend beyond the prairie's borders, who so far holds a license to follow his conscience in Washington as long as he delivers for his constituents back home.

His national stamp is a double-edged sword in South Dakota, which voted against him for president in 1972 and then reelected him to the Senate in 1974.

"Part of it is grudging respect," said Loila Hunking, the state Democratic chairman. "People may not like a position he takes, but when it's just them and the lever in the voting booth, they think about the stature he's brought South Dakota, they think in a little broader terms, and they vote for him."

But his long absence from South Dakota over the years, coupled with the political wear-and-tear of 24 years of balancing conflicting interests and the inevitable argument for new blood, have taken their toll.

And that's to say nothing of the raging unpopularity of President Carter and the general grumpiness of farmers in this heavily agricultural state of roughly 600,000 people. Asked how 1980 differs from 1974, when he last ran, McGovern cited two things: disillusionment with government and the fact that a bushel of wheat was selling for $5 then and is selling for $3.50 now, while farm costs have risen dramatically.

McGovern has been supporting Sen. Edward M. Kennedy (D-Mass.), who narrowly won the Democratic presidential primary here last month but is not considered widely popular in the state as a whole.

McGovern's vulnerability became readily apparent in his own Democratic primary contest, when a little-known conservative supported by right-wing and antiabortion forces got nearly 38 percent of the vote against McGovern. McGovern had never been tested before within the party that he built almost single-handedly during the 1950s, and his challenger, Larry Schumaker, a mathematics professor who moved to Texas 20 years ago and returned only late last year to enter the primary against McGovern, had been expected to get no more than 25 to 30 percent.

"If they wanted to hit us on the head with a two-by-four to get our attention, they succeeded," said McGovern's administrative assistant, George Cunningham.

As of now NCPAC has suspended its anti-McGovern advertising and has added Sen. Thomas F. Eagleton (D-Mo.) to its senatorial hit list. But it plans to return to South Dakota if its surveys show that McGovern is recovering. "My suspicion is that everyone who could possibly dislike McGovern now does so," said Dolan of NCPAC, which has already spent at least $114,000 to defeat McGovern (the Democrats contend the total from all New Right groups in higher than that).

Dolan denies that NCPAC has pulled out because of a backlash against its messages. Local politicians say there has been some backlash, but not much. "Some people say they're fed up it [NCPAC's campaign], but not as many as I'd hoped there would be," McGovern says.

By comparison with NCPAC's barrage, the McGovern-Abdnore clash is a mutual exchange of powder puffs -- a tough-guy, nice-guy sequence of routines that strikes the McGovern camp as no mere happenstance, although Abdnor denies any collusion.

Abdnor says McGovern is out of step with South Dakota, and McGovern calls Abdnor an unwitting tool of "big business" -- pretty standard stuff for a classic liberal-conservative confrontation in the Midwest. What makes this year different is not only the preconditioning of the state by NCPAC but also the fact the Abdnor, one of the state's two House members, is probably the strongest contender McGovern has faced in a senatorial career of election squeakers.

Abdnor, the 57-year-old son of Lebanese immigrants, is an earnest, rough-hewn, straightforward dirt farmer from Kennebec in the arid, sparsely populated mid-section of the state. He is so unpretentious that he begins his officials campaign biography with this self-summary:

"A few weeks after Jim Abdnor arrived in Washington . . . he encountered the kind of cultural shock one might expect from a South Dakota farmer in the big city. While attending a formal dinner party, Abdnor stared out a picture window at the city's monuments and famous buildings, its blanket of bright lights, and said to someone standing nearby, "This sure beats a Saturday night at the Kennebec Corner Cafe.'"

Abdnor says he likes McGovern and admires many of the things he's done for the state, but calls him too liberal for South Dakota voters, who, he says, are increasingly concerned about distinctions in defense and economic policies and will thus view McGovern's national positions more critically.

Abdnor bristles at suggestions that, if he wins, he might owe his election to NCPAC. He denies allegations in a Democratic complaint to the Federal Election Comission that he and NCPAC have conspired and thus cannot be considered as having separate campaigns. But he does not go so far as to disavow NCPAC's efforts on his behalf.

Two straws in the wind: McGovern is introduced at the Alexandria, S.D., centennial parade as "South Dakota's senior citizen" and is rudely heckled by some young people. A day later, he walkes into a bank in nearby Yankton, and the bank president writes out a $500 check for his campaign. Like its weather, said a local reporter, South Dakota's policies "can never be taken for granted."