On the entirely related subjects of politics and human nature, Fallon is the smartest guy I know. And nobody else is even in second place. Of course Fallon is a Democrat, but there is no recorded instance of Fallon's party preference ever interfering with either his candor or his canniness.

Before the Reagan administration's last week of March began to dissolve into tantrums and territorial wars, Fallon phoned. What follows, in language considerably less colorful than the original, is Fallon's spring political analysis.

In December 1980, Ronald Reagan violated a respected rule of presidential Cabinet-making. The rule, which Reagan surely knew, absolutely forbids the selection for the Cabinet of anyone who wants to run for president. For the president who breaks that rule, there is a mandatory sentence unrelieved and public political migraine. Now Reagan is paying dearly for that trespass.

The logic of the rule is compelling. A president, at the minimum, must be able to assume that his own political agenda is also his Cabinet's. That's not always the case when one or more Cabinet members -- and their dedicated staffs -- are scheming as well as dreaming about the 1984 Iowa precinct caucuses, now only 33 months away.

Take the case, suggests Fallon, of the current secretary of state, Alexander Haig, a man with no critical shortage of ambition Secretary Haig and his staff would undoubtedly agree, if asked, that the Reagan administration will prosper or perish on its handling of the economy. Interest rates and the president's popularity cannot both stay up: One must drop. All the energy, attention and effort of the administration will be required to win congressional passage of the Reagan tax and spending cuts. It's as simple as that.

Fans of Haig would insist that ambition and loyalty sometimes co-exist, and they can even collaborate. After all, Reagan's victory also heralded a major toughening in our dealings with the Russians. An argument could be made this way: The Soviets only grasp any message when it is accompanied by real force. What better place to demonstrate our renewed national will than in our own hemisphere?

Now Fallon begins to anger. Aid to Nicaragua had already been suspended by that notorious softie (by the Reagan-Haig standards) Jimmy Carter. Before Ronald Reagan was inaugurated, economic and military aid to El Salvador had been resumed, and the great rebel offensive there had collapsed. There are probably, by actual count, more "guerrillas" in the San Diego Zoo than there are in El Salvador.

But El Salvador was big news. It pushed the Reagan economic plan off the front pages and pushed Al Haig onto the cover of Time. In 1979, Time had anointed John Connally, the Fortune 500's homecoming king, as our next president. Now in 1981, from the very same people, we were treated to a cover photo of no-nonsense Al Haig looking like the prize graduate of the Billy Martin assertiveness school.

Another large problem about El Salvador directly involves the presidency of Ronald Reagan. Throughout 1980, American voters expressed one recurring doubt about a President Reagan. That doubt, helped along by Carter campaign attacks, began with the fear that, as president, Reagan might recklessly plunge the United States into a foreign disaster. Candidate Reagan successfully resolved those fears, but El Salvador recycles them.

If you are advising Al Haig about 1984, then, according to Fallon, you would have to remember the last Republican secretary of state at the 1976 Kansas City convention. There a big majority of all the delegates in the hall and virtually every conservative in the city willingly and openly expressed their rage toward Al Haig's principal sponsor, Henry Kissinger. Most of the Republicans at Kansas City believed that Henry had betrayed them and us, especially in our dealings with the Soviets.

No Republican can realistically seek his party's presidential nomination in the face of united conservative opposition. Henry Kissinger may very well be a tactical genius, but for anyone seeking the next GOP nomination, he is almost surely a political liability. So, the argument would go, it would be important early on to draw the differences with Kissinger, to send a message to both Leonid Brezhnev and Sen. Jesse Helms. And what better place to do it than El Salvador?

That's all from Fallon.