For several reasons, the odds are that Ronald Reagan will be a one-term president.

* He may choose not to run again.

Already he's the oldest man ever to serve as president. Understandably, he might not relish beginning the rigors and frustrations of a second term at the age of 74. His recent actions in rejecting advice to alter his economic program, as urged by so many counselors in and out of the White House, suggests that he is staking all on this term.

* He could run and be defeated.

Until very recent years presidents who sought another term were given it by the voters. In the modern age of American politics only one sitting president seeking reelection had been defeated: Herbert Hoover in 1932. Then, beginning with the 1976 campaign, two straight incumbent presidents, Gerald R. Ford and Jimmy Carter, TIME 6 were turned out of office. Such a back-to-back rejection of presidents by the voters had not occurred in this century.

For Reagan, the prospect of being similarly rejected is no longer implausible. Not long ago he seemed invincible. Yet he finished his first year with the lowest job rating, as measured by the polls, of any president at a comparable time in office since the creation of the Gallup Poll in 1934.

In the past two weeks I have met voters in Minnesota, Texas and California who are becoming disillusioned with the president's performance and are turning against him. This was no scientific sampling of public opinion, but these were people who had been strong Reagan backers. Such chance encounters, from such different segments of society, makes me suspect that the president stands in greater trouble today than is generally recognized.

* He could run afoul of the currents of recent history.

As the political scientists are quick to point out, we live in a time of sudden convulsions where the old norms have been broken and the public becomes accustomed to rapid change. Presidents unable to survive two full terms are part of the story of present-day life. Whether for reasons of death, disgrace or defeat, whether for circumstances beyond their control or of their own making, five straight presidents before Reagan--John F. Kennedy, Lyndon B. Johnson, Richard M. Nixon, Ford and Carter--have been denied the opportunity to serve two four-year terms in the White House. The precedent for continual presidential turnover clearly is well established.

I recite this mournful history not out of feckless speculation about the nature of Reagan's fate, or to try to read his mind on whether he wishes to run again, but because reality raises a larger question for this president.

With good reason, debate about the Reagan presidency has centered on the nature of his bold, if not radical, prescription for change. In terms both of the nature of government and the working of the economy he is forcing the nation to examine fundamental aspects of American life that seldom are truly seriously addressed.

Reagan deserves great credit for compelling citizens, through the political process, to decide what kind of country they want, with which values, and what they are prepared to pay for in the expenditure of national treasure and labor. I am not saying his course is correct, wise or fair, but, unlike many presidents, he is engendering a great debate about great issues.

The stakes, of course, are immense.

In the last few days two commentators have put these stakes in clear perspective.

Dr. John E. Hansan, executive director of the National Conference on Social Welfare, speaks from the vantage point of the government and social policy:

"President Reagan has prompted the American public to reconsider the role of government in our free enterprise system," he wrote. "By proposing to reduce federal support for some social welfare programs and to leave others entirely to the responsibility of the states, the president appears to want the nation to return to conditions that existed prior to World War II.

" . . . If the Reagan administration is successful in further reducing federal support for social programs it is certain it will have severe and far-reaching effects on present inter-governmental relations and the financing of social programs at state and local levels. In the short range, many citizens in need of services or benefits will be unable to obtain what they require to be independent or productive--hurting people will suffer even more; some preventable social and health problems will become acute or chronic, requiring even more time and resources in the future for repair, restoration or rehabilitation; and the proportion of federal funds spent for our poorest citizens will shrink while the proportions going to middle and upper income citizens will grow larger."

Another view, delivered by a political consultant, Horace W. Busby, to Louisiana bankers, deals with the impact of the president's budget deficit on the nation's social fabric.

"If those projected numbers are not addressed," Busby said, "if response is to be simply acceptance, the consequence could be an unimaginably trying, difficult and uncertain time ahead. This deficit, coming at this time, from this administration, has the potential for precipitating a political crisis of grave magnitude in this country, in Europe and elsewhere in the world.

"If, however, the numbers are addressed responsibly, if the Congress, the country and the chief executive respond courageously, in a spirit of constructive cooperation, great good can finally come from what would be, otherwise, a moment of great peril."

As he says, "The choice is exceptionably stark and clearly drawn."

How these issues are resolved provides reason enough to make this president's first term fateful, but there's another. It involves a great issue not being debated. Reagan's presidency bears directly on it.

The issue is public service.

Along with the destruction of so many presidencies has been a steady erosion in the concept of public service. Partly, this has been the corrosive price of years of doubt and dissension growing out of the unpopular Vietnam war abroad and the Watergate scandal at home. But it also has stemmed from the cumulative attack on people who serve the public, especially those in government. Too often they have been made the scapegoats for failure to solve complex national problems. Too often the attack on them has been led by politicians seeking the highest office.

The result has been predictable: the best are increasingly driven out of government. When public esteem is low for those in public service, fewer talented people seek such a career. That situation has been growing worse. In the long run it is the nation that pays the price for failing to attract and hold its most capable people.

Of our recent presidents, none exudes a greater sense of old-fashioned patriotism than does Reagan. None speaks with more personal warmth and in a way that commands attention. He retains a great opportunity to rekindle faith in the political and economic system. If he is flexible instead of doctrinaire he can help set the nation on a new, more productive course. If not, we are entering a graver crisis flowing out of another presidential failure.

For Reagan, the question is not whether he will be the latest in a line of one-term presidents. The question is what he will do with the time remaining of the term for which he has been elected.