If it's Tuesday, it must be Salvador - or maybe Sri Lanka?

Much is made these days of the hectic life of a presidential candidate - the umpteen trips to Iowa and New Hampshire and all stops in between. It almost seemed symbolic when the pipes froze in Walter Mondale's house while he was courting votes as far away as Puerto Rico.

Home is where a candidate goes only when it's time to collect more votes at primary time in his own state.

From now until November, there will be the usual groaning over the irrationality of a system that so prolongs the democratic process for bystanders as well as participants. Wives of candidates will testify to feature writers on the trials to domesticity of the nomadic life of office-seekers. As always, Professor So-and-So will suggest a six-year term for presidents, while Professor Such-and-Such will recommend the parliamentary vote-of-confidence arrangement common to other democracies.

After November, the matter will be dropped for another four years. But the Problem of the Endlessly Peripatetic Candidate does not disappear with election. Less often do we take note of how much rushing about goes on after our leaders are in office, and now we are including appointed officials as well - the whole movable feast known as an administration.

One can begin documenting the perpetual motion with the No. 2 men. Vice-presidents seem to think they can prove the importance of their job by one major criterion - how many trips they take outside the country. We don't want to know the statistics - they might frighten us - but each vice-president appears to break the previous vice-president's record, without necessarily improving the prestige of the office. George Bush of late has turned into a veritable sonic blur - now touring Central America, now holding a press conference in Moscow.

A secretary of state used to live at the end of a long row of cables - the buck stops here (stop). But since the example of Henry Kissinger, the physical presence of a secretary of state in another country has become his ultimate act of negotiation. Shuttle diplomacy has evolved into a general law, so that the appearance of George Shultz on a Brazilian golf course constitutes a chapter on Latin American relations.

The logic - if it can be called that - runs deep. We are, according to our social chroniclers, a nation of nomads, led by the greatest nomads of them all. By our current code of values, we appear to have made two hitherto neutral pastimes the automatic measurements of great achievement: travel and memo-writing. Once a trip had to be shown to accomplish some purpose. Once a memo or report had to lead to specific action. No more. Now one travels to conferences or conventions that exist simply to be traveled to, and nobody questions that progress has been made there - especially if a memo is written or a report is filed.

We have all become, in one sense, our own secretaries of state, giving the phrase ''on the road'' new dimensions. What self-respecting politician, from dogcatcher up, can run on a record of staying-put after the lieutenant governor of Massachusetts has found it within his duties to tour Europe, doing research on the continental approach to acid rain?

Supreme Court justices may be the last powerful officials who can afford to stay in one place, as nearly everybody but ambassadors used to from the time of George Washington until, roughly, World War I.

What does it mean that our leaders build their reputations by being outside Washington - by living on the run?

Would Thomas Jefferson or James Madison have been able to write something as profound as the Declaration of Independence or the Federalist Papers if they had been scribbling in the cabin of a jet, 30,000 feet up? There are some exercises of thought and judgment that require quiet, uninterrupted attention before a desk, not a pull-down plastic tray - with our feet literally on the ground.

Even if our leaders stay in one place, enough hopping around will go on just to keep up mentally with the flash-fires all over the globe. The tempo of history cannot be slowed down.

But the tempo of our responses can be slowed down - at least in some cases - and should be. The decisions that affect our survival are in danger of being made by bleary-eyed travelers in a state of partial disorientation, possessing more power and less time for deliberation about its uses than any other leaders in history.

As life grows more complex, the practical and moral issues that constitute politics at home and abroad demand more rather than less study and reflection. Human beings maintaining frantic fly-about schedules are not going to be able to answer 1984's questions.