TAMPA, FLA. -- Every presidential campaign encourages the conceit that the moment is at hand for a fresh start. Candidates always have a need to convince voters that their election, in and of itself, will make a great difference to the individual and the nation. Otherwise, why bother to vote?

When an elderly president is completing his second term, as is the case this year, it is particularly tempting to adopt the metaphor of ''new beginnings'' or, at the very least, of ''change,'' as most of the White House aspirants have done.

Yet history shows that except in times of true national crisis -- of war, depression or civil strife -- continuity is more the hallmark of successive administrations than radical redirection.

The analyses of President Reagan's budget, sent to Congress last week, noted the limited nature of the ''revolution'' he has been able to achieve in his years in office. If ever a man came to Washington with the explicit determination to alter the status quo, not simply to preside over the existing structure, it was Ronald Reagan. And the success he achieved in his first year in cutting taxes and redirecting spending from domestic to defense accounts signaled the seriousness of that commitment.

One should not underestimate the significance of the changes Reagan has accomplished in both the 1981 and 1986 tax laws and in his budget spending priorities. Nor should the alteration in the U.S.-Soviet relationship or the shift of initiative for domestic policy making from Washington to the state capitols be overlooked.

But the commentaries on this budget were correct in emphasizing, as Paul Blustein and Tom Kenworthy did in this paper, that ''Reagan's budget for his last year in office envisions a federal establishment that, in its broad outlines, is remarkably similar to the governmental structure he found when he came to office in 1981.''

Viewed as a share of the gross national product, overall federal spending is the same as in the year Reagan took office, while federal revenues are down less than 1 percent. Within the budget, entitlements are the same, national defense and interest payments are each up roughly 1 percent, and discretionary domestic spending is down 2 points. The major building blocks of the welfare state -- Social Security, Medicare and Medicaid, for example -- have all grown during the Reagan years.

All this does not prove Reagan's insincerity or ineptitude; it simply measures the constraints on any president from the fundamental commitments and policy decisions made and ratified in the years before he took office.

What is true of Reagan is almost certain to be even truer of his successor. As Blustein and Kenworthy wrote, ''Reagan's impact on the shape of government is certain to last far beyond his tenure, making him in some ways a three-term president.''

The main reason is that the Reagan legacy will include not just a largely undiminished welfare state and a vastly enlarged defense establishment but a budget deficit and federal debt of a scale no previous incoming president has faced.

In the newly published volume ''Challenge to Leadership: Economic and Social Issues for the Next Decade,'' from The Urban Institute, a Washington think tank, editor Isabel V. Sawhill writes that ''budget deficits have all but ruled out new spending initiatives or tax subsidies to accomplish various public purposes.

''Indeed, at least part of the next decade will have to be spent getting the nation's fiscal house in order, leading to a search for existing commitments that can be curtailed or new sources of revenues. . . . Changes in spending and taxes of the sort that might conceivably be within the bounds of current political feasibility {no matter who occupies the White House} will simply not do the job. A new national consensus to do the currently unthinkable {such as raising income taxes or cutting Social Security} will have to be forged.''

One possible mechanism for helping forge such a consensus exists: the bipartisan National Economic Commission, chartered by Congress late last year, is just now starting work on a report on the deficit problem that is due next March 1, just five weeks into the new president's term.

You can think of that 14-member panel as the National Fig-Leaf Commission, on the theory that its report will allow whoever is in the White House to say, ''Golly, this deficit problem is a lot worse than I ever understood. I guess I'll have to modify some of the promises I made during the campaign.''

But no commission will do the job by itself. The challenge for the next president will be a daunting one: to recognize the severity of the constraints that are in the Reagan legacy; to be realistic about the discipline they will require; and to be bold enough and skillful enough to persuade the public to take the short-term actions that may be necessary to make it possible, despite those constraints, to start moving toward vital, long-term national goals.

Those are exceptional demands. Talking change is easy; achieving it, for the next president, will be very difficult.