Can either Ronald Reagan or Jimmy Carter add?

That question seemed likely to plague the 1980 presidential election campaign a few weeks ago as the two major candidates traded jibes -- and took turns trying to defend themselves -- on their proposals for reviving the economy.

Over the months-long campaign, Reagan had propounded a spate of costly new tax-cut and spending plans that analysts said would be impossible to enact together and still balance the budget, as the GOP candidate was contending.

At the same time, Republicans were finding flaws in Carter's economic and spending proposals that appeared to be fostering doubts among some voters about how viable the president's program was.

Last week, the two candidates finished laying out revised election-year economic packages that rejuggled the numbers so that they finally add up but left serious questions about how fully either set of claims should be believed.

Although Reagan's advisers finally got his numbers to jibe satisfactorily, the calculations were based on heady economic growth estimates that analysts believe could not be sustained for long without roaring inflation.

And while Carter's economic package seemed less assailable on the issue of its basic assumptions, critics questioned whether it contained much more than campaign rhetoric.

Meanwhile, independent presidential candidate John B. Anderson continued along with a hodge-podge of still-unfinished tax-cut and spending proposals, without even trying to portray them as part of a formal economic program.

The new move by Reagan last week represented a clear shift -- a victory for main-line Republicans, who had sought to convince Reagan to adopt a more traditionalist approach.

There were two reasons that Reagan's numbers had not added up before: First, as candidates often do early in a campaign year, the Californian simply had proposed too much in spending and tax breaks to fit anyone's budget.

Secondly, his adherence to the economic philosophy epoused by Rep. Jack Kemp (R-N.Y.) resulted in distortions in the way he computed the estimates of how much his proposals would cost.

As a result, Reagan was genuinely vulnerable. Carter was arguing repeatedly that it would be impossible to cut taxes and boost defense spending as sharply as Reagan wanted and still balance the budget, as the governor also advocated.

And both Republican and Democratic economists supported that view.

What Reagan did last week was to pare back his tax-cut program. He put a ceiling on his proposals for defense outlay increases and specified new spending cuts.

The result was to close off the opportunity for Carter and the Democrats to dismiss his program as outlandish, while retaining the basic tax-cut plan he has hacked from the start.

Here's what the GOP candidate did:

TAXES: Reagan retained the huge tax cuts for individuals he has been advocating since the start of the campaign, but shucked -- at least temporarily -- a lot of the more costly proposals that had been weighing down his economic program.

By far Reagan's most significant step was to jettison his earlier endorsement of the costly "10-5-3" business depreciation plan and back the far less costly proposal that has been drafted by the Senate Finance Committee.

That alone pared some $30 billion from the cost of the Reagan program.

The candidate also shelved, pending further review, his costly proposals to repeal the newly enacted, crude-oil tax, enact a tuition tax credit, repeal the estate and gift tax and exempt savings account interest from taxation.

The savings from these moves were enormous.Before Reagan's changes, the cost of his tax-cut package would have zoomed to $293.3 billion a year by fiscal 1985. His cutbacks pared this to $192 billion -- a reduction of $101.3 billion.

DEFENSE: Despite the rhetoric about sharply higher defense spending, the GOP candidate pledged an overall limit on military increases last week that would keep total Pentagon outlays to credible levels.

The ceiling would hold overall defense spending to less than $300 billion by fiscal 1985 -- well above the $248 billion that Carter currently is projecting but not far above the $270 billion forecast by the Senate Budget Committee.

SPENDING CUTS: Until last week, the GOP presidential candidate had been vague about spending reductions. Reagan talked generally about the existence of a massive $50 billion in government waste but refused to say how much he'd cut.

Last week, Reagan came up with a two-part plan for holding down spending that managed at once to relieve any fears that he'd meat-axe the budget unmercifully, while still not giving opponents any room to criticize specific spending cuts.

He first endorsed a plan by former Nixon administration budget director Caspar Weinberger to trim a modest 2 percent from projected spending in fiscal 1981 and 1982 and one percent a year in fiscal 1983-85 -- simply by cutting back on "waste" and fraud.

At the same time, he also served notice he'd "like to" cut another one to 9 percentage points each year on top of taht, gradually bringing the cutbacks to a full 10 percent less than what outlays otherwise would be under Senate projections.

"You just can't know what you'll cut until you get into office," a Reagan adviser insisted.

THE DEFICT: Reagan put off his promised budget-balancing until fiscal 1983 rather than trying to eliminate the deficit immediatley, as he had proposed in his earlier program -- bring his plan back into the realm of reality.

The earlier budget-balancing pledge had stemmed in part from the idologically based assumption that massive tax cuts now would stimulate economic growth enough to wipe out the budget deficit the year after the tax cuts took effect.

Although Reagan's rejuggling effort blunted any outside criticism that his economic plan didn't add up, the candidate still left some serious questions about just how viable his program is:

Reagan's long-term estimates added up largely because they were based on optimistic estimates of how rapidly the economy will grow in coming years -- and projections of continued rapid inflation -- keeping revenues high and outlays low.

(The economy's actual growth rate has been far below the one Reagan assumed would occur. And analysts caution that trying to sustain the Reagan growth rate for long inevitably would exacerbate inflation further.)

Despite warnings from traditional economists, the GOP candidate still insisted he would want to go through with his massive tax cuts whether Congress approved his spending cuts or not -- a move economists say would worsen inflation sharply.

For all of Regan's assurances of sizable budget surpluses in coming years, such projections tend always to be on the optimistic side. Carter has relied heavily on these, as well.

Even with last week's revisions, the GOP candidate's economic program still contrasts sharply with that of Carter, who, ironically, has proposed more tax breaks for business and far less tax relief for individuals (see chart).

Carter proposed a new "economic renewal program" Aug. 29 containing a more modest overall tax cut and a spate of new programs and incentives designed to encourage business investment.

However, the program -- which was put together hastily earlier last month -- is regarded as mostly election-year window-dressing even by many Carter officials.

And Carter's long-term projections suffer from the same set of uncertainties as Reagan's. Except for the modest tax cuts the president has proposed, analysts generally see little in the plan apt to have much impact.

Independent candidate Anderson has no formal economic plan as such -- only a series of small proposals for increasing the exclusion for interest and dividends and eventual automatic adjustment of tax rates for inflation.

(Estimates worked out with the help of Anderson staffers are included in the charts.)

What the latest backing and filling has shown is that Carter and Reagan can get their estimates to make sense arithmetically if they're pushed enough by public opinion.

Implementation of their respective programs is, of course, an entirely different matter.