The signs of incipient candidacy seem obvious. Mario M. Cuomo, the Democratic governor of New York, makes his first trip to Moscow, then impresses the establishment lions at the Council on Foreign Relations with his analysis of U.S.-Soviet relations. Aides ask political reporters what kind of feedback they got on the address and inquire -- oh, so solicitously -- how bad the damage from the ''tapes incident'' seems to be for Massachusetts Gov. Michael S. Dukakis.

After declining scores of political invitations, Cuomo accepts bids from Democratic organizations in several states. He even profits, some boosters say, from the acquittal of Geraldine Ferraro's husband, John Zaccaro, on extortion charges. You can be a politically connected Italian American in Queens and still be clean, Cuomo's friends trumpet.

So the stage is being set for a late entry into the 1988 Democratic presidential race by the magnificent orator who has fascinated his party since the keynote address of the 1984 convention, and who broke so many hearts when he said he wouldn't run in 1988. Right? Wrong.

I say that, not because I know what the Albany Enigma has in mind. I definitely do not. Mario Cuomo is too smart, too subtle, too good at word games for me to decipher. Interviewing him is a game he always wins.

But I think it extremely unlikely that circumstances will provide the opening the governor would need for a dramatic last-minute rescue mission. Not impossible, for nothing is impossible in the bizarre nominating system we now have. But extremely unlikely.

The plausible predicate for a Cuomo rescue mission would be a stalemate, in which no avowed candidate has gained a significant edge. The odds against that are very heavy.

Why? Because the six candidates who are out there keep getting better. I saw three of them -- Dukakis, Rep. Richard A. Gephardt and Sen. Albert Gore Jr. -- this past week in New Hampshire. Every one of them has improved his stump speech markedly since I last heard him. The same is true in the tapes and television I've seen of Sen. Paul Simon and former governor Bruce Babbitt. And Jesse Jackson was darn good on his feet to begin with.

One can make a reasonable guess at the script of the Democratic contest; it's just that we don't know who will play the leading role. My guess is that as the Democratic candidates continue to fine-tune their messages, in debates and in personal appear-ances, one of them will finally get his rhetorical radar locked on the target vote in the Democratic primaries and know that he's got it right in his sights.

These are not slouches. You can see them moving in on the target. And to assume that all six will miss completely defies probability. My guess is that by the first of the year, when most of the members of that Democratic primary audience start paying attention for the first time, someone will have the right message ready for them.

If you want to know what will happen then, think of Gary Hart in the first three months of 1984. All through 1983, even this late in that year, Hart was floundering around, testing one theme after another, looking for the message that would work. When he found it, you could feel the spark jump from candidate to constituency, and he took off as if he were jet-propelled.

The difference between 1984 and this year is that there is no Walter Mondale in the Democratic field to bring the highflier back to Earth. Mondale could do it -- barely -- because he had accumulated vast resources of organizational and personal loyalty over the years.

Except for Jackson, no one now running in the Democratic contest has that kind of large, loyal constituency. The others' fans are but recently recruited and, in the vast majority of cases, judging from those I've met, could shift to another candidate without much regret if it turns out that the other candidate has ''the winning message'' for the Democrats.

Jackson does have a real constituency, with passionate commitment. But everything we know at this point suggests that constituency is substantially less than a majority of the Democrats' primary electorate.

The likely scenario, therefore, will see the other five Democrats testing their messages in Iowa and New Hampshire this winter and discovering who has the best fix on that electorate. Those who are off target at that point will have neither the resources of their own nor the attention of the media to keep them in the race for long.

My hunch is that someone will emerge pretty quickly as the alternative to Jackson, and we will see a series of 65-35, 60-40, 55-45 contests. The high man in those primaries will be the Democratic nominee.

I don't see when, where or how the opening for Cuomo appears. Which probably proves how myopic these old eyes really are.