The clock ticks terribly fast for any president of the United States. The fiscal 1983 budget Ronald Reagan will present to Congress next week is the first he and his associates have had the opportunity to shape from beginning to end. And when that budget year expires, the 1984 presidential campaign will be only three months away.

The breakneck pace of the presidential term is part of the explanation for a wave of unease that has come over the Republicans and the conservative movement, as they await the formal unveiling of what is rightly considered the policy and political centerpiece of any administration--its budget.

The budget will reveal in stark terms what Reagan managed to pass over with only fleeting comments in his State of the Union address: the unhappy fact that this conservative government confronts deficits brushing the $100 billion- a-year level for the rest of this term and beyond.

What the Wall Street Journal called "the baleful effects of big deficits" are only the symptoms of a greater doubt gnawing at conservatives. That is the fear that runaway government and a sick economy may overpower Reagan's remedies.

Last Friday, as a succession of orators tried to galvanize the members of the Republican National Committee into a show of enthusiasm for the mid-term political campaign, copies of Friday's Wall Street Journal were being passed from hand to hand.

The effect was like being doused in a cold shower. "As they wrestle with a recession that wasn't supposed to happen," the Republicans read in Ralph E. Winter's lead story in their favorite newspaper, "some businessmen are starting to think the unthinkable--that Reaganomics might never bring the promised prosperity."

The fear that deficits and high interest rates will choke off the promised turnaround may prove unfounded. But, as if by coincidence, a number of conservative publications and pundits are giving voice to a feeling of extreme nervousness about where Reaganomics may be taking the Republican Party and conservatism.

Kevin Phillips, who ever since the late 1960s has been periodically proclaiming an "emerging Republican majority," said in the latest issue of his newsletter, The American Political Report:

"There's a growing feeling in conservative and New Right circles that the Reagan economic and policy shortfalls shaping up for 1982 threaten an ideological and electoral crisis. . . . Important elements of the Reagan coalition, not least conservatives, are breaking ranks and moving towards a save-yourself politics."

Horace W. Busby, a conservative Democrat and former Lyndon B. Johnson aide, wrote the clients of his Washington consulting firm a stiff, four-page critique of the leadership and policies of the Reagan government. He said it reflected the comments of "men of substance, experience and proved influence," many of them "long friends of Mr. Reagan."

Not long ago, Busby was writing about a long- term Republican "lock" on the electoral college and the presidency. But in his latest memo, he reported widespread fear that the momentum of the conservative thrust, which extended from 1978's Proposition 13 through the 1980 Reagan- Republican sweep and the legislative triumphs of the first eight months of 1981, "began to stall" in the final quarter of last year.

It began to stall not just because of recession and rising unemployment, but because Reagan and his associates seemed uncertain about how to master the forces at work in the economy and the world, he said. Both Busby and Phillips noted that in December, Reagan's standing in the polls reached what Busby accurately called "the lowest level ever recorded for any president in the same year as his inauguration."

They fear an economic-political unraveling. If investors lose faith in Reaganomics because of soaring deficits, the job-producing recovery may never come, or may be quickly aborted. By November, disillusioned voters may snatch away the provisional mandate they gave Reagan and the Republicans in 1980 by electing waves of Democrats to governorships and congressional seats.

None of that is certain, of course. But the very fact that these fears are being voiced in these places at this time shows how thin the surface of patience, confidence and consensus supporting the Reagan program may be.

The credibility of that program will be tested in the reaction to the Reagan budget. The president, budget director David A. Stockman and the rest will be judged on their candor-- and there is little time for recouping if that credibility is lost.

This is a fateful fortnight for Reagan--and the country.