AESOP, THE FABLED political sage, may or may not have had President Carter's reelection committee in mind when he wrote: "We would often be sorry if our wishes were gratified." Certainly, after the Illinois returns were in, no one would argue that the president's supporters did not get what they had openly and fondly hoped for: the apparent elimination of Sen. Edward Kennedy as a serious challenger to Mr. Carter and of all serious challengers to Ronald Reagan as the 1980 Republican presidential nominee.

Sen. Kennedy's problems are now obviously neither demographic nor geographic. He lost Illinois just about as badly as he had lost two entirely different states -- Iowa and Florida. Mr. Kennedy and his supporters must live with a very unpleasant fact: for a majority of Democratic primary voters, he is evidently an unacceptable alternative to Mr. Carter. It is no longer a case of big states or small states, Catholics or Baptists, farmers or factory workers. Mr. Carter may not be terribly popular with a majority, but Mr. Kennedy is terribly unpopular with it. That much seems clear after eight primaries.

But with Mr. Kennedy out of the race, as he seems likely to be soon, Mr. Carter will be robbed of his weekly triumph at the polls. Voters will probably turn their attention to different numbers, like the inflation rate, the interest rate, the price of gasoline. In fact, possibly the worst thing that could happen to President Carter politically would be the withdrawal of the one "opponent," he has consistently been able to defeat, and whose challenge has brought him broad support.

The other half of the wish-fulfillment may present an even more serious problem for Mr. Carter and his plans for reelection. Ronald Reagan has now won primaries in New Hampshire, Vermont, South Carolina, Florida, Georgia, Alabama and Illinois. At this time in 1976, he had not won a single primary. The only realistic hope that his surviving challengers, George Bush and John Anderson, now have is that Mr. Reagan will commit an egregious error that will make him, overnight, unacceptable to Republican voters. This could happen, of course. But we would remind those two gentlemen that Mr. Reagan twice won the governorship of our most populous state, where the Democrats outnumber the Republicans 8 to 5. He is no rookie at the fast political track.

If, as it now appears, the nomination races are basically over in both parties, then we are about to embark on an almost eight-month general election campaign that will be something new (and may become something old) for all of us. There is also the possibility of the development of a plague-on-both-your-houses independent-party movement. We are willing to wager that before Nov. 4, the Carter partisans will wish more once that the Illinois results had not been quite so definitive so soon. The Reagan folks will probably wish the same thing.