The contenders for next year's Republican presidential nomination are so convinced of an ultra-conservative trend in their party that nine out of 10 of them are now directing their appeals almost exclusively in that direction.

They could thus end up splintering the right-wing vote to such an extent that the one candidate appealing to party moderates might well make a more impressive showing in the primaries than the supposed composition of the GOP suggests.

That could turn a moderate-to-progressive dark horse like Rep. John B. Anderson of Illinois, for 10 years the respected chairman of the House Republican Conference, into a horse of another color.

This could not have been foreseen a few months ago, for it was then assumed that there would be others vying for the moderate vote, particularly such prominent figures as Sen. Howard Baker, former ambassador George Bush and former president Gerald Ford. More recently, however, these hopefuls have all been moving to the right in an effort to match the kind of conservatism exemplified by rivals such as Ronald Reagan, John Connally, Sen. Robert Dole, Rep. Philip Crane and others.

The upshot has been to leave Anderson the only "pure" moderate in the race, with a consequent windfall of publicity and growing attention. He's beginning to arouse the kind of curiosity that an even-darker horse, Jimmy Carter, ultimately exploited so successfully.

It's hard to say whether the competition will keep on playing into Anderson's hand, but the other contenders seem to be stepping up their conservative approach.

Who could have imagined a short time ago that George Bush, a quintessential "establishment" figure, would go to such lengths to appease party extremists as to resign from the ultra-respectable Council on Foreign Relations and make a production of it? The council, while suspect in right-wing circles, has long been a citadel of safe and sound New York bankers and corporation lawyers.

Who could have guessed that Geral Ford, who initiated SALT II while president, would court the hard-liners, too, by turning against the arms-limitation treaty, even though it is basically (with some improvements) much the same agreement that Ford and Henry Kissinger worked out earlier with the Russians?

Sen. Baker's opposition to SALT II also was a disappointment to those who expected him to give it the kind of bipartisan support he accorded the Panama Canal treaty. The hostile reaction in some GOP quarters to his statesmanship on Panama, however, apparently convinced him that he had to shift to the right to be a viable contender for the Republican presidential nomination.

In contrast, Anderson continues to support SALT II as outspokenly as he opposed the Vietnam War. Moreover, he has dared to go against the conservative grain in backing the Equal Rights Amendment, the Humphrey-Hawkins full employment bill, civil rights, public financing of political campaigns and open housing. He didn't hesitate, either, to attack a Republican president in the Watergate scandal. He is, though, an eloquent fiscal conservative.

The first presidential primary in New Hampshire is made to order for a candidate in Anderson's special position. It's the state that gave smashing primary victories to a moderate Dwight Eisenhower over Sen. Robert A. Taft ("Mr. Republican") and to Henry Cabot Lodge over Sen. Barry Goldwater, the hero of the right wing. Another early primary is in a state, Massachusetts, where Nelson Rockefeller won a write-in victory over Richard Nixon in 1968.

After New England, Anderson plans to concentrate on his home region in the Midwest. It is encouraging to him that the voters in Illinois, Michigan, Wisconsin and Iowa have elected Republican governors -- James Thompson, William Milliken, Lee Dreyfus, Robert Ray -- who ran as moderates. Gov. Milliken has said more than once that it is not only "morally wrong but politically stupid" for the GOP to embrace an ideology so narrow as to shut out all but a vocal right-wing minority. It will be interesting to see what, if anything, he does for Anderson.

Not so long ago, when he was less politically active, Jerry Ford referred to Anderson as "one of the most able people I know." Ford also said, "John is very articulate. He's attractive, and he's a little more middle-of-the-road or liberal than other candidates. John could surprise some people."

That about sizes it up.