The economists have a term for it: opportunity cost — the benefits forgone when an investor puts his capital into one project rather than another. His choice may prove profitable, but another choice might have been even more so — and so he’s lost the difference between the two. That’s the opportunity cost, and it can be measured not just in dollars but in time or energy or anything else of value.

Politicians, like the rest of us, make much the same mistake when, given a chance to score political points, they seize the moment and exploit it for all it’s worth, or rather for what they think it’s worth. Actually they might gain something incalculably more by declining the opportunity to engage in a little cheap drama — and instead serve their fellow citizens by raising the level of public discourse, and win a place in history. That is true greatness.

There will always be those who think it’s foolish to miss any opportunity to lambaste the opposition. Their philosophy: A soft word turneth away the voters. Every chance for a sound bite must be seized.

Let’s hope there will also be those who try to rise above the fray to see farther, think more clearly, and act more honorably.

It’s the difference between a ring-tailed roarer like Howard Dean — the perpetual and now professional partisan — and a quiet thinker like Connecticut’s Joe Lieberman, who’s willing to speak unwelcome truths even to his own, inflamed party. And be willing to pay the price for it in a party primary.

It’s the difference between a Joe McCarthy and an Adlai Stevenson. Let it be noted that Governor Stevenson paid the usual price for thoughtfulness and eloquence in a televised age — he lost his race for the presidency in 1952.

(And in 1956, too, by which time he’d learned the cost of talking sense to the American people and was content to just repeat catch phrases, which availed him even less.) But in the presidential campaign of 1952, he was still introducing novelties like reason and eloquence into, of all things, an American presidential race.

Some criticized Adlai Stevenson that year for “speaking over the heads of the American people” when he was only trying to get us to look up. Looking back, it’s even clearer that one needn’t have agreed with the gentleman from Illinois to admire his faith in the American people, and in the power of reason.

When a red-in-the-face Bill Clinton tells off an interviewer on Fox News, he may fire up his party’s long frustrated base, and win the plaudits of those partisans who made up their minds long ago. About everything. But his little tizzy cost him more than his dignity. While reveling in the chance to tell off his critics, he lost an opportunity to raise the level of public discourse.

Amid all the finger-pointing rage and the kind of selective history that’s good mainly for rhetorical purposes, reason evaporates. Bill Clinton’s attack was followed predictably enough by counterattacks, and what might have been a meaningful debate about the future gave way to one more rehash of the past. Given an opportunity to address the next generation, the former president seemed interested only in making points for the next election, or maybe the one after that. Which seemed the extent of his vision.

Much the same goes for George Bush when he responds to provocative questions at his news conference not by trying to raise the level of discussion but by going after the questioner. The president is less than presidential at such moments. With the result that the case he should be making — the case for going after terrorism on its home ground, for expanding democracy in the Middle East, for victory instead of drift — goes unmade.

And so another opportunity to raise the level of public discourse is lost, replaced by partisan slogans. Labels take the place of thought: cut-and-run, stay-the-course … fill in your own cliche. Meanwhile, the appeal to reason goes unmade.

Other leaders have appealed to high principle at other critical times, well knowing the price they would pay. Nevertheless, they chose to pursue the opportunity to make a difference in history, to shape it rather than be shaped by it.

Think of the despised Churchill of the 1930s, that low, sordid decade, who dared warn of the gathering storm even if it meant he would be ignored — at least until he was sorely needed.

Think of the then unpopular Harry Truman, who chose to stand fast in Korea (“Truman’s War”) rather than either withdraw or turn that conflict into a world war while his presidential term drained away in frustration.

Think of how Ronald Reagan was ridiculed and railed against when he foresaw a world without the Soviet Union, and dared call that regime the evil empire it was.

Even in the midst of a congressional election that promises much heat, little light and even less honor, every press conference, every public appearance, every political speech presents every national leader with an opportunity to raise the level of public discourse. Each time our politicians choose to debase it instead — in order to please the crowd, or just to serve their own egos — they pay the cost. They lose the opportunity to mark the history of these times with their honor.