President Reagan and Walter F. Mondale have spun a series of conflicting myths about Reagan's record in their battle for the White House, each beaming to the American people broad messages that vastly oversimplify the complex reality of what Reagan has done since 1981.

Every campaign produces such myths, from John F. Kennedy's claimed missile gap in 1960 to Reagan's assertion two decades later that he could balance the budget, cut taxes and increase defense spending all at once.

Both the missile gap and the balanced budget disappeared once the candidate became the president.

This year's crop has focused largely on the Reagan domestic policy record -- one that the president hails as a forward-looking revolution and that Mondale decries as an unfair step backward.

At times, both candidates perpetuate the same myths but for different reasons.

For example, Reagan has portrayed himself as a champion of fiscal restraint, a notion that Mondale reinforces when he complains about unfair budget cuts in social programs.

What Reagan really accomplished was a shift in budget priorities, not an overall reduction. Federal spending in Reagan's term reached a bigger share of the nation's gross national product than at any time since World War II.

Both Mondale and Reagan have helped to preserve the myth that the Reagan tax cuts were extraordinary by past standards. The reality is that Reagan's across-the-board tax cut was a historical correction that only brought tax burdens back to the levels just before the big inflationary spurt of the late 1970s. Reagan then took back about one-third of his original tax cut in later tax increases.

Reagan insists repeatedly that the huge federal deficits can easily be melted away by a combination of economic growth and further spending cuts. But his senior analysts, and most of those outside the government who have looked at the problem, say this is impossible. If reelected, Reagan will face hard fiscal realities that bear no resemblance to the easy solutions he has described in the campaign.

Mondale perpetuates the myth that Reagan's defense buildup is excessive and can be reduced. The reality is that lower inflation and congressional cutbacks have trimmed the Reagan defense budget to levels not much higher than what President Jimmy Carter called for in his final year in office, when Mondale was vice president.

Reagan also perpetuates the myth that he wants government to intrude less in people's lives. But while he follows this principle on economic issues, he has abandoned it on many social and moral issues.

Reagan signed a bill forcing states to raise their drinking age or lose some federal highway aid, a direct use of federal intervention. He fought for and signed legislation in which the government forces high schools receiving federal money to open their facilities to religious as well as nonreligious groups. Reagan also would move to alter the government's role in abortion and school prayer.

The myths of the 1984 campaign center on the tax and spending issues that Reagan put at center stage from his first day in the presidency.

Reagan fosters the myth that he has reduced the budget, telling audiences that he trimmed the rate of annual growth in government spending from 17 percent to 6 percent and declaring that he intends to further reduce spending as a share of GNP.

The flip side is that Reagan also tries to make spending a virtue when he wants to refute charges that he has hurt the poor. "We can show that in every instance and in every program we are spending more money and helping more people than ever before in our history," he says. And, he insists, it is "just not true" that "we have been trying to balance the budget on the backs of the needy."

Reagan doesn't point out that spending on programs for the needy would be less if Congress had given him cuts he sought.

Mondale has complained about the unfairness of these domestic budget cuts, from Social Security to education and job training programs. But Mondale's budget plan includes only a selective restoration of such cuts and suggests that he would follow a general path of fiscal restraint -- albeit with different priorities than Reagan -- in trying to meet his goal of reducing the deficit by two-thirds over five years.

Reagan has refused to put out a detailed fiscal plan until after the election. His critics and supporters say he has shifted the debate on spending but his performance is mixed.

He pledged to shrink the domestic side of government and did reduce its after-inflation growth rate from about 5 percent a year in the late 1970s to less than 1 percent from 1980 to 1984. He also succeeded in fulfilling his campaign promise to shift the mix of federal programs away from domestic spending and toward defense.

But he failed to meet his goal of reducing federal spending to 19 percent of GNP, partly because of the recession. Spending has hit 24 percent of GNP, and many Reagan advisers say the best they can hope to do is get it back to 21 percent.

For all his boasting about reducing the rate of growth in spending, the overall increase this fiscal year will be about 5.5 percent after inflation -- higher than in any of the Carter years.

Reagan opened his presidency promising that domestic spending cuts would "not be at the expense of the truly needy" and saying he would be "evenhanded" by trying to cut business subsidies. But many of the subsidies survived while domestic spending cuts hit many of the government's programs for the poor.

According to Gregory B. Mills and John L. Palmer of the Urban Institute, Reagan's efforts to restrain nondefense spending were "significant," but he got only about half of what he asked for from Congress, and that was "far less than he needed to control the deficit in the face of the large tax cuts, the defense buildup and the economy's performance."

Mondale has charged repeatedly that Reagan's defense buildup was excessive and said he would trim it; Reagan has claimed that his predecessors "unilaterally disarmed" while he alone put the defense buildup into place.

The reality is more complex. The defense buildup began in the late 1970s when Carter pledged to NATO a 3 percent real increase in defense spending. Under election-year pressure, Carter later sought even more; Congress approved big increases just before Reagan took office and still higher levels after he became president in 1981.

In the last three years, Congress slowed Reagan's buildup against his will. Sharply reduced inflation also had the effect of bringing the total Reagan defense level down. The result is that Reagan's buildup is only a fraction higher than what Carter projected for fiscal 1985.

The tone of Mondale's rhetoric has shifted somewhat this year as circumstances dictated. When he was fighting for the Democratic nomination, Mondale said of Reagan's defense buildup: "We're building every conceivable kind of weapon, and we're spending, the defense budget, in excess of anything that makes sense or anything that's necessary to our defense." But in the fall campaign against Reagan, Mondale said in his Sept. 10 budget speech in Philadelphia, "Meeting the Soviet challenge requires a steady, sustainable increase in defense support. That's not the Reagan binge -- but it's not free, either."

In his September plan, Mondale proposed defense spending increases of 3 to 4 percent a year after inflation, only somewhat less than Congress has been giving Reagan. Mondale's differences with Reagan are less on the defense budget's size and more on its makeup. Mondale says he would put more emphasis on preparedness and less on procurement.

Reagan's 1981 tax cut has become similarly enshrined in the campaign mythology. Both candidates tend to exaggerate the size and significance of the tax cut to suit their purposes -- Reagan to take credit for the economic recovery and Mondale to suggest that "average income families . . . got no relief from Mr. Reagan's tax cuts."

Reagan won a three-year, 25 percent, across-the-board cut in individual income tax rates, indexing of tax rates to inflation and a number of corporate tax cuts. He later accepted mostly business-tax increases that offset more than a third of the original cuts, but indexing and rate cuts have survived.

In one important respect overlooked by both candidates, Reagan's 1981 cuts were part of a trend. Congress frequently cut taxes every few years to compensate for inflation pushing taxpayers into higher brackets.

But the 1977 and 1979 cuts did not fully offset the effects of the dramatic rise in inflation, and federal personal income tax burdens reached record levels in 1980. Social Security payroll taxes were also climbing.

"It is not surprising that this confluence of factors and the highest total tax take in postwar history led to irresistible pressure for tax cuts," analyst Joseph J. Minarik says in the Urban Institute's new study, "Federal Budget Policy in the 1980s."

Reagan seized that pressure to put through tax cuts that returned tax burdens approximately to the levels of the late 1970s. He later accepted a big tax increase in 1982; accelerated Social Security payroll tax increases in 1983; a gasoline tax increase and another tax hike this year as part of the deficit "down-payment" plan.

The president also accelerated a continuing shift in the composition of federal tax revenues. Corporate and excise taxes have been declining as part of the overall mix, Social Security and other payroll taxes have been rising and individual income taxes remaining about the same.

Yet another aspect of this year's campaign mythology is Reagan's 1980 pledge to get government "off the backs" of business and consumers through deregulation and elimination of some government agencies.

He did achieve some regulatory "relief" by relaxing enforcement of existing laws, but his term did not produce major regulatory legislation that would make lasting changes. In some cases, such as environmental rules, there was such a backlash to Reagan's initial efforts that the administration retreated.

In economic deregulation, Reagan accelerated the efforts of previous administrations to lift regulations on such industries as banking, airlines, securities, telecommunications and railroads. But the White House also bowed to political pressure and held up a trucking deregulation bill at the behest of the powerful Teamsters union.

In his stump speech this year, Reagan says that "one of the first things we did was decontrol oil prices . . . ." But he fails to point out that decontrol of oil was largely accomplished by Carter at great political cost; Reagan only accelerated the final stage.

Nor was Reagan successful in rolling back all the government agencies he once targeted for elimination -- the departments of Education and Energy still remain. For the most part, Reagan did not eliminate entire programs but instead whittled away at them.

Reagan, who made a career out of attacking federal bureaucrats, kept his pledge to trim the government's work force. There are 97,456 fewer civilian workers in the executive branch than when Reagan took office.

But like so much else in his presidency, there is another side to the story: There are also 79,206 more civilians working for the Defense Department than when he became president.