RONALD REAGAN'S greatest political asset is not his sincerity or his handsome face or his polished ability to perform in person or on television. No, Reagan's secret weapon and the chief reason why I believe he will be our next president is the consistent tendency of his opponents to take him lightly and underestimate him.

It began in California in 1966 when Democrats howled with glee at the notion of a mere movice actor seeking the governorship. The voters, especiallly blue-collar Democrats, didn't see it that way. Reagan skillfully depicted himself as a sensible and moderate conservative and pinned Democratic Gov. Edmund (Pat) Brown to the liberal extreme as a reckless spender who had brought the state to the brink of bankruptcy. Reagan won by a landslide.

In 1970, he was reelected easily, defeating Jesse Unruh, the California assembly speaker and the "Big Daddy" of Sacrament political pros. Unruh learned too late the painful price of overconfidence. He paid Reagan, whom he bitterly disliked, this grudging tribute: "As a governor I think he has been better than most Democrats would concede and not nearly as good as most Republicans and conservatives might like to think. As a politician I think he has been nearly masterful."

In other words, the politican twice elected to lead the most populous states has the rare gift of appearing to remain an amateur while beating the pros at their own game. Reagan blandly declares: "I'm not a politician by profession. I am a citizen who decided I had to be personally involved to stand up for my own values and beliefs." His opponents swallow these soothing words and begin counting their votes before they are cast, as many are today.

In part, this situation reflects the monolithic eastern influence over print and electronic political journalism and opinon-molding. Although Reagan has been a national figure for almost two decades and has a broad, enthusiastic base of popular and organizational support, he remains to most Easterners, including the politically informed a stranger and a sterotype: an aging right-wing ideologue whose nomination would surely set the stage for a repeat of the 1964 GOP debacle behind Barry Goldwater.

That kind of shallow, unthinking perception has already begun to wane, and it will continue to fade as the campaign progresses. Easterners, like everyone else, will learn the differences between 1964 and 1980.

Unlike Goldwater, for example, Reagan avoids hypothetical questions that create controversies for no good reason. He also is smart enough to realize that he cannot allow himself to become the issue, that he must make Carter's incompetence and the resulting dangerous condition of the country today the uppermost concern in voters' minds, as it should be anyway. (There is, to put it mildly, no shortage of evidence to press the case.)

In part, too, Reagan is mistaken for an ideologue because he launched his career in the 1964 campaign, making a televised speech describing his personal pilgrimage away from New Deal liberalism and toward Goldwater Republicanism. But, elected governor of a state in which Democrats vastly outnumber Republicans, he proved his ability to govern by recruiting the best available appointees without regard for party labels, by successfully bargaining with the Democratic-controlled legislature, and by concentrating on fiscal and institutional reforms that would outlast him. After eight years, he left California with several useful innovations and a tidy half-billion-dollar budget surplus.

One of the landmarks of his tenure was the Welfare Reform Act of 1971, which his admirers claim saved the state an estimated $2 billion. The most impressive accolade came from his successor, Gov. Jerry Brown, who in the 1974-75 recession declared: "The Reagan welfare program is holding up, and considering today's high unemployment, it is amazing that it has kept welfare down as much as it has."

Rather than the cliche notion that he is an "unelectable right-winger," then, Reagan will increasingly be seen as spokesman for a socially responsible and informed mainstream American conservatism whose time has come.

Once we get past the sterotypes -- and I suspect that any Gerald Rafshoon attempt to promote them in Carter media ads would badly backfire -- we are left with some rather persuasive personal and political reasons, I think, why Reagan is likely to be the next occupant of the Oval Office.

I have been working with Reagan since 1975, helping advise him on issues and strategies, which you may wish to take into consideration in what I say. But I can attest from our time together that Reagan possesses those qualities essential to political success and effective leadership: inbred caution, an instinctive sense of priorities, a practical flexibility, a willingness to compromise on less important matters, an aversion to needless risk-taking, and, above all, a crisp, calculating intelligence that seldom strays for long from the issue at hand.

Reagan also has an undervalued but indispensable quality every good politician needs: a genuine sense of humor, the ability to laugh at himself and the absurdities cast up by the democratic process. A sense of humor is not only important personally but for the bridge of trust it creates to those people who do not think in terms of abstract issues, which includes most of us. Did Reagan panic at the prospect of Former President Ford belately entering the GOP race and snatching his prize away? Not at all. He confidently quipped: "I guess Jerry has developed a slice."

I do not suggest that Reagan will soft-shoe his way to the presidency with humor and his Technicolor smile alone. I do say that he will continue to show the spontaneous and appealing common sense that have marked his appearances in televised campaign debates thus far.

But beyond these personal qualities, Reagan will be elected. I believe, for essentially three other substantial reasons.

First, he has wider appeal than any other Republican to traditionally Democratic voters, expecially lower-middle-class families and ethnics. Note that he did exceptionally well in working-class neighborhoods in New Hampshire and Chicago.

Second, he attracts Democrats and independents with his blend of social-cultural conservatism and economic populism and activism, expanding the base of the Republican Party and making future control of at least one chamber of Congress a realistic GOP ambition.

Finally, he sees that with inflation rocketing toward 20 percent and the overall tax burden on the ordinary American household the heaviest in peacetime history, tax cuts make sense. He sees the strong parallel with his 1966 opportunity when he seized the state-level economic issue. Now he can attack President Carter as the architect of national bankruptcy.

Our presidential election process, it must be remembered, is controlled by the middle of the electorate, the least "political" elements within it. These voters are nominally more Democratic than Republican (largely for inherited, nonideological reasons). But in fact they are independent, volatile and tied to no program or party. They operate on the basis of an unspoken consensus that the American economic system and political culture are in working order -- or at least so they could assume until recent years. Now their distrust of the system, of the culture and of their flawed, failed leaders is becoming total.

In the electoral process, the two major parties form rallying grounds for activists and platforms from which the nominees bid for the favor of those in the middle, those who decide elections on the basis of personality and subjective emotions more than politically defined issues. They simly choose a person they trust. "Moderation" is obviously a precondition of success in this process, and Reagan knows it. What's more, it is a comfortable stance for him. He still thinks and talks like a man who came out of a working-class Democratic household in a small Illinois town.

In a now-forgotten but important speech telecast on July 6, 1976, for example, Reagan expressed some of these Middle American themes that have since become even more powerful and will be heard again next fall.

"I was once a Democrat myself and believed that party represented our values faithfully," he declared. I don't believe I changed. But the intellectual and political leaderhsip of the Democratic Party changed." He went on to say: "The Americans who keep this country going -- the ones who fight the wars; drive the trucks and raise the kids; the farmer and fireman, craftsman and cop, they are wondering -- for the first time -- if the governmental institutions they have upheld and defended really care about them or their values."

It was no accident that Reagan, whose established political base is in the South and West, officially launched his candidacy in the Northeast, making his announcement speech in New York City and then visiting a union hall in Boston and a blue-collar suburb in Philadelphia. The significance of this symbolic foray was twofold: First, Northeastern Republican opinion, never as "liberal" as imagined, has mellowed considerably toward Reagan since 1964, along with the entire county, which has moved perceptibly rightward; second, and more important, as witness last week's campaigning in New York and Connecticut, the Reagan effort will not concede any supposedly "safe" Democratic turf anywhere, not even in the Deep South or in the northern and midwestern metropolises.

By contrast Jimmy Carter, despite the advantages of incumbency, remains an outsider to key Democratically inclined ethnic and religious groups beyond Dixie. And he has problems there, too, among conservative Southerners who thought Carter was one of them. In 1976, Ford, it will be recalled, actuallly beat Carter, 52 to 48 percent, among white Southerners. Reagan will do even better. Indeed, Reagan has the best chance of any Republican nominee in the postwar era to win over some of the most loyal, hardcore Democrats, including Jews and Catholics.

As Morton Kondracke reported in The New Republic late last year: "What's most politically dangerous for Democrats is that Reagan is on to the smart themes for 1980: productivity in domestic policy and strength in foreign policy. By comparison, the Democrats seem totally confused."

In fact, in the economic realm, Reagan is promising a 30 percent income tax cut over three years to relieve the inflation-caused, crushing burdens of the middle classes -- while Carter is pushing a Herbert Hoover-type austerity program of higher taxes and a fictitiously balanced budget because he refuses to slash much social welfare spending.

Reagan has demonstrated his flexibility to abandoning traditional Republican fiscal orthodoxy in the face of radically changing economic circumstances, expecially soaring inflation that pushes wage-earners into higher tax brackets while reducing their real purchasing power. He now advocates "supply-side" fiscal incentives to increase production, savings and investment largely because of the persistent efforts of Rep. Jack Kemp (R-N.Y.), a onetime Reagan aide in California.

This is part of Reagan's determination to search widely for answers to the nation's problems, and to work with an array of individuals, regardless of the labels they carry, toward the solutions. He enjoys informal, give-and-take issues discussions, later yielding staff-written policy papers. He listens well and asks penetrating questions. After his lengthy sessions with the feisty junior "supply side" economists, Reagan tested their unconventional advice by consulting such experience veteran policymakers as former Treasury Secretaries William Simon and George Shultz, former Budget Director Caspar Weimberger and former Chief White House economic adviser Alan Greenspan.

Until he was abruptly fired on the eve of the New Hampshire primary, strategist John Sears totally stage-managed the candidate and intimidated most of the Reagan staff. Sears has been succeeded by a group of rather relaxed Californians led by quiet-spoken Ed Meese, a San Diego lawyer who, like others on the present low-budget staff, once served Gov. Reagan. Although they are naturally protective of Reagan's schedule and energies, for he expects to be treated as top Hollywood stars once were, they are entirely open to counsel and assistance from all points of the Republican compass as well as from sympathetic Democrats.

Contrary to the right-wing stereotype, Reagan is an instinctive coalition-builder. This grows out of his long experience as head of the Screen Actors Guild, which involved complex studio negotiations. Much of the intellectualy vitality and polticial effectiveness of a future Reagan administation would come from diverse, unexpected sources, including the so-called neoconservatives whose philosophical roots are in the right wing of the pre-Vietnam Democratic Party. Democrats and independents attuned to Reagan's pragmatic approach to governing would be needed at every level of his administration, as they were during his governorship. He would rely heavily on able, experienced Democrats in shaping his defense and foreign policies. He would make an early and sustained effort -- as the newcomer Carter did not -- to gain bipartisan congressional support for a program to unify the nation, stablize its collapsing economy and restore its security abroad.

Overall, the thrust of a Reagan presidency would be toward a new political realignment and an enduing bipartisan coalition of moderate conservatives.

From our conversations, I know Reagan has ideas for reorganizing and revitalizing outmoded parts of the executive branch. For example, he probably would use his Cabinet in an entirely new way, based on his successful experience in California. Cabinet members would no longer be captives to their bureaucracies and represent traditional social, political and fiscal constituencies at the White House. Instead, they would be expected to represent the president's policies and priorities to their parochial bureaucracies, demanding concrete responses to new policy signals.

The presidency itself would be reshaped by Reagan's extraordinary skills as a communicator using television to inform, persuade and lead. This is not mere "image-making" but the essence of democratic government in the age of the mass media. In our presidential system, if the chief executive does not clearly articulate the nation's goals, set priorities and urge consistent means toward these ends, the result is the cacophony and confusion of the Carter presidency and the near-paralysis of government.

The question that naturally follows all this is: Where are the electoral votes to give Reagan victory next November?

They are not to be inferred or projected from current opinion polls, for these fail to tell us who will actually vote and where. In 1976, immediately after his nomination, Carter held a 30-point-plus lead over Ford in the national polls, the biggest in history. In November, he barely squeaked through, gaining 297 electoral votes (minimum required: 270) to Ford's 240.

As political analyst Kevin Phillips instructed us several years ago, America has become "balkanized," broken up into several increasingly antagonistic regions and subregions, where cross-cutting, divisive forces -- economic, energy, racial and ethnic, social and culture -- work powerfully against the conventional unifying appeals of politics. By far the strongest of these appeals is the time-honored negative one: Let us join together and throw out the rascals who have made such a mess of the economy, foreign policy, national security and practically everything they've touched.

If Reagan can succeed in turning the election into a plebiscite on Carter's record and the condition of the country, simultaneously establishing himself as a responsible moderate conservative alternative, his potential electoral votes are almost everywhere, including Carter's shaky Southern citadel.

Carter won the presidency in 1976 by reclaiming the Old Confederacy (except Virginia) from the Republicans; and he might well lose it in 1980 by having several Southern states wrestled from him. He does not have dependable compensatory strength elsewhere, even in traditionally Democratic areas such as the Northeast and the industrial Midwest, at least not against an opponent such as Reagan who may score unusually well among Democratic voters in these areas.

Political analysts of widely differing persuasions agree that 1980 ought to be "a Republican year." But not even the conservatives among them, such as Phillips, are willing to predict that Reagan is that successful Republican. Everything will depend, they say, on the unpredictable ebb and flow of events, on Carter's luck and Reagan's health and on the popular perception of who and what Reagan is.

I think, however, that it will also depend on the secret weapon of Ronald Reagan -- the fact that Democrats fail to take him very seriously. At least until it is too late.