Voters of both Parties in this farm hamlet have diagnosed what they think ails Jimmy Carter's presidency as follows: Incompetence and indecisions.

Whether that diagnosis points to terminal political illness, however, is an open question. The president retains a large measure of good will. Thus, although only 16 of our 55 voters rate his presidential performance "good" or "excellent," and 39 say "fair" or "poor," fully one-half say they still have a "favorable" opinion of him as a person.

But the down-to-earth criticism of one retired farmer, a Democrat, hints that time may be running out. "I don't think Carter is big enough for the job." One of 17 Carter voters in our survey of political sentiment here (25 backed Jerry Ford and 13 don't recall or would not say how they voted in 1976), this farmer says he now wishes he had voted for Ford.

Four additional voters who backed the president in 1976 now say they wish they had not, an unusually high defection rate. Only one Ford voter who answered our questionnaire, prepared by Pat Caddell's Cambridge Survey Research, wishes had voted for Carter.

On one major issue after another - inflation, farm policy, handling the Soviet Union, defense spending - our scouting expedition produced failing marks for the president on general grounds of incompetence and waffling. The sole exception: his efforts to settle the Arab-Israeli struggle, which gained solid 2-to-1 support.

Behind that praise may lie a lesson for the president: the political importance of follow-through. Ringing doorbells with three Caddell field representatives on a day of rain, snow and frigid winds on the bleak Nebraska prairie, we found high praise for Carter's consistency in trying to mediate between Arab and Jew. On other issues, he is charged with lacking conviction, receiving less than 50 percent approval even for "trying" to solve the energy crisis.

The dominant theme was struck by a 36-year-old waitress. "If the man would just say something and stick to it!" she blurted out. "That would make a difference." She backed Ford in 1976, but only "as the lesser of two evils." A 35-year-old computer maintenance man, whose job is 30 miles east of here in Omaha, agreed. He voted for Carter and would vote for him again today but called him "too hesitant" on decisions. "There are so many things he said he would do and he hasn't shown that he can do any of them."

By far the major worry here is inflation, along with other economic problems. Thirty-one of our voters listed inflation as the "most important" issue. Indeed, the prevailing lack of confidence in Carter bodes Ill for his new anti-inflation program and might turn ambassador Robert Strauss's silvery hair gray. Only 19 voters "generally approve of the way Carter is trying to handle" the inflation disease (against 31 who disapprove and 5 who do not know). But the president's new anti-inflation program, using Strauss's virtuoso jawbone as his principal weapon, appears singularly unconvincing. Only 4 voters think it will work, 22 say it will not work and 29 are dubious.

Neither Springfield nor Nebraska as a whole represents majority opinion in the United States. This is hawkish country, built on a farm economy. Thus, by almost 2 to 1, our respondents "generally disapprove" of Carter's handling of Moscow. On questions whether he is "too tough" or "to weak" on the Sovits, and whether he is spending "too much" or "too little" on defense, an identical 31 voters say "too weak" and "too little."

As for the farm issue, there was some solace for the president in taking a hard-line position against the "emergency" farm bill defeated in the House 10 days ago under threat of presidential veto. A surprising 24 voters commend him for that, against only 27 who condemn him (the balance unsure). That is a far better mark than he gets on his efforts to cope with the farm situation, and serves to highlight the fact that even here the intractable problem of inflation is overtaking farm prices as an issue. That shift can scarcely be called a political plus for a president who still has not proved his fitness to govern, at least as judged in this small farm community.