In traditionally Republican Nebraska, where the Democratic Party has achieved new heights and is reaching still higher in 1978, party leaders have posted a harsh sign for the Democrat in the White House: Jimmy Carter, Keep Out!

That reflects the midterm election mood throughout the Great Plains, scene of dramatic Democratic gains in recent elections. Farmer unrest, intensified by what Wheat Belt politicians perceive as President Carter's general ineptitude, has raised the prospect of Republican resurgence in the plains states.

Thus, most Carter administration officials - in some cases the president himself - will not be welcome as 1978 campaigners in much of this region. Not only Nebraska but also Kansas, South Dakota, Iowa, Oklahoma and Wyoming are grain states where Democratic candidates for senator and governor fear the Carter imprint.

Unencumbered by such political horrors as Vietnam or Watergate, which generated heavy out-of-power gains for the Republicans in 1966 and the Democrats in 1974, Carter nevertheless has become his own party's worst enemy for 1978.

The clearest case is Nebraska, where popular Democratic Gov. J. James Exon is running for the Senate seat of retiring conservative Republican Carl Curtis. With a general approval rating close to 80 percent after eight years in office, Exon has one waking nightmare: the president.

"Take Carter and the Carter administration out of the equation, and Exon is a sure thing," a leading Republican told us. An Exon aide agreed: "Let Carter set foot in this state between now and November and it is conceivable we could lose." Hyperbole? The shrewdest politicians here do not think so.

Apart from the president's general decline, the White House has shown zealous insensitivity toward this region - as demonstrated April 12 when the House defeated the veto-threatened emergency farm bill. A few minutes after that vote, a leading Nebraska Democrat implored Secretary of Agriculture Bob Bergland to announce immediately the administration's support for a higher, $3.40 target price for hard-pressed wheat farmers, then claim credit for it in the fall campaign.

Bergland, never certain what course his orders from the White House will take, begged off. Anyway, he lamely explained, there will be a higher target price for wheat, and we will get credit for it in the fall.

Instead of hitting the headlines with a dramatic plea for higher wheat prices, Bergland waited a full week. He then sent an aide to the Senate Agriculture Committee, where, in the routine ritual of a committee hearing room, the $3.40 target price was quietly endorsed by the Carter administration.

Such amateurism follows what is perceived here as White House coolness toward Nebraska and the Great Plains. Bergland told NBC's "Meet the Press" last month that "it has been a long time since a Democratic candidate for the presidency carried Nebraska." That "long time" ges back only to 1964, when Lyndon B. Johnson easily won this state.

Bergland went on to say that "if [the voters] choose to reject" Carter farm policies, "so be it." That back-of-the-hand attitude explains Sen. Edward Zorinsky's public ridicule of Carter's efforts to win his support for the Panama Canal treaties. That, in turn, brought staff advice to Carter for a public spanking of the maverick Democratic senator from Nebraska; that would not hurt the president, said the adviser because he has no support in Nebraska anyway.

Indeed, Carter is looked upon as a foreigner, knowing little about Midwestern problems and caring less. Exon was "stupefied" at an Oval Office meeting with the president last July, with other farm-state governors. Carter suddenly said "too much grain" is fed to livestock. Since the major source of income for feed-grain growers is feeding grain to livestock, the governors were dismayed.

Politicians and farmers here have the mistaken belief that Carter's international human-rights campaign is a major factor in declining grain exports. Acrually, delays in shipping grain abroad caused by human-rights restrictions appear minimal - only $10 million in one shipment to Chile, according to the State Department. Nevertheless, the belief has taken hold and is hurting the president.

Thus the paradox of Carter's first off-year election campaign, while evident throughout the Great Plains, is particularly vivid in Nebraska. In a state poised for the first time to send a second Democrat to the Senate and make the Democrats the majority party, the Democrat in the White House is not wanted here between now and November.