In 1976, when he nearly won the Republican presidential nomination after a string of early primary defeats, Ronald Reagan earned the reputation of a long-distance runner who would not quit under adversity. For Reagan and those around him, it is a comforting memory in the wake of Iowa.

But to those supporters who have watched Reagan over many campaigns, including his two successful races for governor of California and his uphill presidential battle last time there are small, disturbing signs that a comeback may be more difficult this time.

One of the signs came on the morining of Reagan's announcement in New York last Nov. 13. Appearing on NBC's "Today show, Reagan was making the point that several world leaders are older than his 68 years when he appeared, surprisingly, not to recognize the name of French President Valery Giscard d'Estaing.

An aide explained later that Reagan knew full well who Giscard d'Estaing was but hadn't heard the question. And since then it has become clear on several occasions that Reagan doesn't hear as well as he use to. At a session in Basin City, Iowa, last Saturday, a question had to be repeated five times before Reagan heard it.

A close aid who said that Reagan is otherwise unchanged from the way he looked and acted four years ago, adds. "The only difference I have noticed is that his hearing has slipped."

There are other little signs. Like most politicians, Reagan has always made his share of misstatements. But they come more frequently now and Reagan rarely seems aware that he has fluffed.

At a New Hampshire press conference last week, Reagan referred to "small, beleaguered Egypt," when he meant Israel. In his statewide live television speech to Iowa voters last Saturday Reagan completely botched a favorite story he has told many times before.

These are small slips that could be made by any candidate. But because of Reagan's age and his reputation as a smooth performer, some see in those slips reflections of a declining candidate.

Reagan's aide observed that he looks and acts younger than his years, and this is frequently true. In the Minneapolis airport late last Saturday night after a 15-hour day, Reagan animatedly swapped stories with reports and members of his staff without any indication that he was tired. w

Out of the campaign trail, however, there are those who have supported Reagan in the past who find his performance on the stump vaguely disappointing.

"He doesn't seem to want to say anyting," complained a Cedar Rapids businessman after a Reagan speech last Novemeber. "I remember him when he could lift the roof right off this place. The old fire doesn't seem to be there now." Reagan's longtime press aide Lyn Nofziger, who quit the Reagan campaign last August after a dispute with campaign manager John Sears, believes Reagan wants so much to be president that he is not saying what he believes.

"They have him so intimidated, so convinced that he shouldn't speak out for what he believes that he's not Ronald Reagan," says Nofziger.

There is some historical basis for this view. In 1976, when Reagan strategists were convinced they had Gerald R. Ford on the ropes in New Hampshire, Reagan did his best to avoid controversial statements. Reagan's subsequent comeback, in North Carolina and Texas was triggered by his passionate declaration of opposition to the Panama Canal treaties.

Now, Reagan is a cautious candidate again who dutifully line up with other Republicans in attacking President Carter's grain embargo after asking him to exercise such leadership. His old supporters would perfer a Reagan who speaks his mind on national issues, stirs his audiences and shows that he would be a vigorous and full-time president.

Whether Reagan can fulfill these expectations is less certain than the returns from Iowa. The signs, however, are that it could be a long road back for Ronald Reagan despite his proven ability to go the distance.