Author Michael Novak, referred to as one of three conservative Republicans in David S. Broder's column last Sunday, calls himself a conservative Democrat.

President Reagan won two major victories last 97-to-3 Senate approval of his tax-reform bill and a hard-fought, 12-vote victory in the House for aid to the contras fighting the Sandinista government in Nicaragua. That ought to be cause for celebration, but in truth, there's a surprising amount of gloom in the GOP at the moment, a sense of -- you should excuse the phrase -- "malaise."

Part of the blue mood can be blamed on reports coming into the White House and Capitol Hill on the Senate races. They raise real concern about Republican prospects of keeping their 53-47 Senate majority in November. The news from key states has not been good, and if it's far too early for defeatism, there is little effort to conceal the nervousness.

But the deeper doubts stem from the sense that time is running out on the Reagan administration, and no one can see what will come next. When I wrote recently that Reagan would be ''a tough act to follow,'' I was thinking of the leadership gap he will leave -- especially as chief of state -- and the legacy of unrepaired budget deficits and unfulfilled expectations on such social issues as abortion, school prayer and pornography.

The last thing I expected was to see that view endorsed and amplified by three such eminent conservative Republicans as Jeane J. Kirkpatrick, former ambassador to the United Nations, Herbert Stein, former chairman of the Council of Economic Advisers, and scholar-author Michael Novak. But when Public Opinion magazine, published by the conservative think tank, the American Enterprise Institute, sat the three of them down for a joint interview in its summer edition, the tone was one of anxiety.

Stein started the discussion in stark terms: ''We're enthralled by illusions,'' he said, adding that many ''emanated from the White House. One was that we could get the inflation rate down without a recession. Another was the illusion that we could cut taxes and raise revenues. Another was that deficits do not matter. Yet another was that there were billions of dollars in wasteful federal government spending that could be cut.''

''We have not established any new lines of policy,'' Stein said. ''We are going to be left with an enormous debt . . . an enormous interest burden we don't know how to manage. . . . We have yet to discover what caused the great slowdown in productivity in the 1980s.'' Looking ahead, he warned that as memories of the double-digit inflation of the late 1970s fade, ''the temptation will be great to reinflate, particularly because of the debt burden.''

Those gloomy thoughts may be dismissed by Reaganites as the mutterings of an unreconstructed Nixon Republican who never believed in supply-side magic. But the cautions from Kirkpatrick and Novak, both of whom have received diplomatic appointments from Reagan, are harder to ignore.

Kirkpatrick said the Reagan administration has ''dramatically improved'' the military balance of power with the Soviet Union and ''has managed to slow dramatically, if not fully stop, the expansion of Soviet influence. That's a very substantial achievement.'' She also applauded the ''spread of democracy in Latin America.'' But she worried that ''Americans are simply not yet persuaded that we should support other people's military struggles'' in such countries as Nicaragua, and added, ''I don't believe there's a general sense of American dominance or leadership in the world.''

The Soviets, Novak warned, ''could give us fits for the next 50 years,'' especially if they decide they are at their peak of power and can only go downhill. ''They have a magnificent set of opportunities,'' he said in countries on their borders, such as Iran and Pakistan.

Of at least equal concern to him, the religious scholar said, is the prospect that ''we may have a period of intense cultural confusion'' in the post-Reagan era. In large part because Reagan embodies and communicates ''classic and traditional American virtues and values and optimism . . . today the country has a good feeling about itself. If, in the next election, we get leaders who are unsure of themselves, that sense may crash against certain rocks.''

I am not certain any of these specific forecasts deserve great weight. But the tone of anxiety in conservative circles and publications is of major significance. Self-confidence is a hallmark of ascendant political movements; it has always been a distinctive feature of Reagan's own approach to campaigning and governing. The catch phrase of his last campaign was, ''You ain't seen nothin' yet.''

If conservatives begin to lose their confidence about the future, that's a loss at least as consequential as the threatened turnover of Senate control. As with the Senate battle, it seems to me that the psychological struggle is one where Reagan will have to rally the troops himself. At every turn, Republicans and conservatives are reminded how much of their future depends on this one man. They need him for everything. And that is asking a lot.