Cruising up 6th Avenue -- yes, I know, it's the Avenue of the Americas now, one of the few mistakes Fiorello ever made as mayor, but it's still 6th Avenue to me -- at the height of the evening rush hour is a scene to be found only in New York. Amid the bikers and the skaters and the joggers and the cars, a young man sails by on skis. He poles his way forward, the roller skates attached to the bottoms of his skis invisible to the throngs milling along the sidewalks. Now and then he raises a pole in salute to the crowds. They wave and shout back.

Good old, big-hearted New York. Once again camaraderie reigns as another crisis grips the city.

That surface display of good will -- or bravado -- is misleading. Beneath the fashionable survive-with-a-smile New York spirit lies a darker strain, one that goes far beyond the latest ordeal for the city and its hapless inhabitants. It is fear.

In the past few days I have heard person after person express the same concerns. Perhaps fear is too strong a term to describe their emotions, but high anxiety and apprehension are not. And it's not the transit strike that prompts their words.

They're worried about the future in ways most of them have never felt before, and they seem to need to talk about it. They volunteer their stories of woe. It matters not whether these are people you've known before, casually or otherwise, or someone you've just met by chance.

Waiting for the elevator the other night, for instance, high over midtown Manhattan, a man in his early 30s rounds the corner, stands before the elevator door, and begins talking out loud as if giving a soliloquy without realizing it:

It was going to be a mess outside, he was certain, and it would get worse before it got better, but he at least was still working, for the moment anyhow. He had less and less money to spend and more and more bills to pay, and he didn't know how anyone else was making it. What a time!

All this pours out without seeming emotion or trace of anger. He is just stating aloud what everyone knows to be true. The very dispassionate nature of his complaint makes it more powerful and disturbing.

What he says sounds no different from the words of others encountered. They sense conditions out of control and growing more grim.

Each day brings more bad news. The prime interest rate moves upward every 24 hours, it seems, making it all the more difficult if not impossible to get home mortgages or obtain bank loans. The inflation rate rises. So does unemployment. Credit curbs threaten harder days ahead for millions. Instability dominates everything. Talk becomes more apocalyptic. Gallows humor is in.

Yet startling paradox exists. In other times, such concerns inevitably lead to political protests. They form the emotional crest that washes out those in power. That has not been happening, as President Carter's continuing demonstrations of strength in the Democratic primaries show.

It would appear that the crucial link in people's minds has not been made between the deteriorating economic conditions and the political responsibility for them. But like the immediate impressions of those convivial New York street scenes, this, too, could be misleading.

You don't hear the anger or the hatred directed at Jimmy Carter these days that Lyndon Johnson and Richard Nixon experienced in their most trying presidential periods. And certainly the subject of Carter isn't on everyone's lips. You have to ask about people's opinions of him; they are not volunteered.

But this should not necessarily bring cheer to the president and his political advisers. For what many people say about Carter carries a doleful political message.

They might vote for Ronald Reagan next November -- and they say that with a sort of personal wonder. They can't believe they're seriously thinking of voting for him. You hear the same things from friends of years' standing. Certified liberal credentials notwithstanding, they could vote for someone whose very name previously was anathema.

As for Carter, he tends to be more dismissed than deprecated. These feelings, of course, can change; everything else has politically all year long. But for now the prospect of President Reagan only brings one more problem for them to ponder. In the face of present conditions, it no longer seems so fearsome.

So the skaters and skiers glide through the streets. They are smiles in the midst of adversity and cheer amid despair. It's like a Roman carnival before they let the lions loose.