Half a score of early-bird presidential contenders are busily bashing one another over ''issues'' sane voters will resolutely ignore for another 14 months.

Can Pat Schroeder, for instance, wriggle to the left of Joe Biden on abortion? Can you stand the suspense?

Meanwhile, everyone's favorite unannounced candidate, Sen. Sam Nunn of Georgia, is being trashed on the op-ed pages as an enemy of the people. His Senate voting record on the issues that enthrall Iowa and New Hampshire activists is said to be insufficiently ''liberal'' for a Democratic nominee.

O, issues, what crimes against political sanity and good government are committed in thy name!

Does no one recall 1980? That was the year the voters expelled from the White House a reckless spender named Jimmy Carter. His challenger promised to drag us back from the brink of national bankruptcy. But by the time Ronald Reagan exits on Jan. 20, 1989, the national debt will have trebled. Such was one big ''issue'' of 1980, and such the aftermath. And by the way, wasn't Reagan the guy who promised to stop babying the mullahs in Iran?

American voters seem to wake up in a new world every four years. For six years we have witnessed a spectacular demonstration of the irrelevance of such ''issues'' as are framed in the glassed-in, hothouse world of such places as Iowa and New Hampshire, months and months before presidential elections.

Little wonder that when it comes to presidents, the country oscillates between extremes of credulity and disillusionment, with little satisfaction between. We vote as ideologues and one-issue fanatics when nominating presidential candidates. But then we collapse in shock and fury when the real world forces presidents, once elected, to disregard the silly things they said when courting caucus votes on a snowy night in Iowa.

The Iran-contra affair is much to the point. The Reagan administration's daffy policy of arming the Iranians was, of course, a circus of maladministration. It also betrayed at every point the macho attitudes on terrorism and hostage-taking that Reagan articulated at Carter's expense in 1980. This exercise in self-contradiction offers a vivid lesson in the compelling need for skepticism about the so-called issues.

After all, it's a pretty incompetent political poseur who can't posture to suit a gullible fanatic. But the structure of politics is such as to discourage predictable thinking by any serious elected official.

When the voters stop being infatuated by ''issues'' -- at least of the more inconsequential sort -- Sam Nunn's prospects will improve. And if they do, and he decides to run, the prospects for a serious presidency after Jan. 20, 1989, will also improve -- drastically.

That Nunn's voting record (he has, for instance, supported aid to the contras) doesn't suit knee-jerk liberals and single-issue zealots in every particular is entirely irrelevant to his presidential qualifications and his likely appeal to serious voters in a general election campaign.

Nunn is a man of character, and if he runs and is elected, one would expect him to stick to his guns. But the perspective of the White House is unique; it is very unlike that of the Senate. Moreover, one would expect of him a competent, seasoned and systematic approach to policy making. In the long run, the second qualification matters far more to the well-being of the republic than parroting the glib shibboleths that win votes in obscure caucuses and backwater primaries.

Think back to the ''issues'' that have figured in the making of some of our most distinguished presidents. Lincoln was regarded as a crypto-abolitionist and Theodore Roosevelt as a hotdog from a dude ranch. And Franklin D. Roosevelt was the candidate who claimed, quite solemnly, that the Great Depression had been caused, or at least worsened, by Herbert Hoover's reckless spending.

To recall such nonsense is to recall how little ''issues'' really matter in the election of presidents -- by comparison with intellect, seasoning, character and judgment.

That is why some of us are waiting -- and praying -- for Sam Nunn to ignore the self-appointed gatekeepers and take the plunge.