The overture to President Reagan's State of the Union address was provided by some of his earliest and most ardent conservative supporters. In a series of press conferences and television appearances, these gentlemen, who share not just an ideology but a tendency to premature plumpness, quivered their young jowls and said that Ronald Reagan was straying from the cause of righteousness.

In a musical metaphor, they were bassoons. The temptation was to snicker a bit, as one laughs at the bassoon bleatings of Grandpapa warning young Peter that if he plays in the woods, he may be eaten by the wolf.

But a more apt musical comparison was suggested by Jude Wanniski, one of the guardians of supply-side orthodoxy. When asked about the mood of the conservative activists, who gathered here last week to mark the anniversary of Ronald Reagan's inauguration, Wanniski said, "It was a feeling of heartache."

Immediately, the lyrics of the old ballad came to mind: "Heartache. Heartache. My love for you means only heartache." And on the flip side, of course: "You always hurt the one you love, the one you shouldn't hurt at all."

The criticisms from the true-faith conservatives about Ronald Reagan fall into that rich literature of lovers' laments. They really cannot understand why the man who led them by hand from the exile of the post-Goldwater years to the promised land of the presidency should be asking them to abandon so many of the dreams they all shared.

Reagan's heresies may not seem major to most voters, who, the polls tell us, worry most about keeping or getting a job and managing the mortgage on a house. But if you have attended 16 consecutive annual banquets of the Young Americans for Freedom, where Ronald Reagan has burnished the dream that free Chinese will some day overthrow the Communist regime in Peking, it is hard to swallow your leader denying advanced jets to the "liberators" on Taiwan.

If you have heard Ronald Reagan say year after year that when prayers were banished from the classroom, discipline diminished and SAT scores began their decline, then it is hard to abide his presidential silence on the issue.

When you know that Reagan knows, in his heart of hearts, that Henry Kissinger was a Rockefeller snake who tempted Richard Nixon into doing business with the Russians, then it is hard to abide Ronald Reagan embracing that Nixon-Kissinger prot,eg,e, Alexander Haig, and allowing him to negotiate with Gromyko.

The true-faith conservatives cannot accept the possibility of personal betrayal on Reagan's part. They cannot even abide the notion that the presidency may have changed his thinking. It is more than politics that prevents their making a clean break with him, although it is a fact of life that he has his own grip on their constituents. "They like him," acknowledged Moral Majority leader Ron Godwin, "and they are pleased he is where he is."

Beyond all that, they have too many of their own hopes invested in him to end the affair. It would, quite seriously, break their hearts.

So, they do what comes quite naturally to them: they invent a devil theory to explain why Ronald Reagan is being seduced from the path of righteousness. It is, they say, the mischief of the presidential advisers who were chosen--in the words of conservative fund-raiser Richard Viguerie--"on the basis of their experience and credentials," rather than their adherence to conservative ideology.

They have in mind people like White House chief of staff James A. Baker III, communications director David Gergen, congressional liaison chief Kenneth Duberstein and others in the State, Defense and Justice departments who served on Capitol Hill or in what Viguerie characterized as "failed" Republican administrations.

It may seem a curious sort of conservatism that rejects the importance of experience and credentials. But one of the favorite conservative writers, M. Stanton Evans, declared in last week's Human Events that with "the dumping of (national security assistant) Richard Allen and the departure of (political aide) Lyn Nofziger . . . the administration of Ronald Reagan is close to being captured by 'moderate' Republicans who opposed his presidential aspirations. The White House staff itself is heavily tilted toward the followers of George Bush and others of even more liberal persuasion, while the management of foreign policy is firmly in the grasp of those who came to power as acolytes of Henry Kissinger.

"Here and there, some pockets of Reaganite resistance remain," Evans concedes. But neither he nor anyone else attempts to explain how this "takeover" has been possible, unless Reagan himself is either a dupe or so detached from his own office that he is oblivious to what is happening.

That far, they will not go. But there is more than disillusionment being expressed here. The faithful are preparing their alibi for the possibility of Reagan's presidential failure. The fault lies not in him, they are saying, nor in the beliefs they share with him. It is "those others" who have done him in, and made the White House the conservatives' Heartbreak Hotel.