Every presidential campaign needs its Louie, and here, threading his way through the maze and noise and maddening congestion of midtown Manhattan, he is.

Louie, I should explain, is the sort of guy George Wallace used to like to cite as the true average American. If you want the real dope about what people are thinking, Wallace would tell reporters traveling with him when he was seeking the presidency, talk with your friendly, salty, blunt, outspoken and often bigoted "Everyman" of a local cab driver. In short, check it out with the Louies of America.

Now the Louies never were all that good a barometer of public opinion, any more than that mythical "man on the street" provides a guide to what the country thinks about anything at any time. But when it comes to this election, this particular Louie seems worth listening to, especially in his assessment of the two presidential candidates and how his view of them has changed.

For Louie, it was the debates -- all three of them -- that made the difference. And the point of this column is that it's not only Louie whose thinking has been affected by those debates. Deep concerns have been aroused in surprising quarters. They promise to linger long after the election.

But, first, about Louie:

He is Milevoj Lucijan ("they call me Louie"), a dark-eyed man with a drooping black mustache and a shrewd but haggard and worn look that makes him appear much older than his 43 years. He came to the United States from near Trieste, Italy, 18 years ago and began working at a Bronx machine shop. It was a good job, "real nice," but the shop went out of business. Louie began driving a cab. He's been a New York cabbie for 15 years.

Louie pays close attention to politics, and he's a voter. "Neither one of them's going to help me," he says, referring to Ronald Reagan and Walter Mondale. "How are they going to help me? I got debts you wouldn't believe, and that's not going to change no matter who's president."

Louie likes the Democrats, but in recent years he's been disaffected from the self-proclaimed party of the people. Last time, he couldn't bring himself to vote for Jimmy Carter. Too weak. This time, he's been undecided. Now, after watching all the debates, he's made up his mind.

"Reagan's old, and he looks old," he says. "They say he sleeps in meetings. He's 73, and he looks it. He'll be 77 when he goes out, if he makes it. I didn't like what I saw on TV. I don't like what it looks like for another term. Mondale, he says he'll raise the taxes. I don't like that, but anybody in there is going to have to, and at least he's honest. Reagan, they say he's borrowing $480 million a day and we owe $500 billion more than we owed before. So what's he gonna do about that? He don't say."

Louie was warming to his subject. What really got him was the contrast on television between George Bush and Geraldine Ferraro.

Louie's not a great fan of his fellow Queens resident, Ferraro, but he came away from the vice-presidential debate solidly in her corner.

"They say ass and bitch and things like that about her. Why they say that? What's the matter with them? What are they afraid of? That's not strong. That's weak. I don't like it. My wife don't like it. My 15-year-old daughter don't like it. I'm voting for Mondale and Ferraro."

So, he says, is his wife. And so would his daughter, if she could. But one member of the family dissents.

"My son says he's voting for Reagan. We try to talk him out of it, but no good. He's 20, and he's going to night school to get a degree. He says Reagan got him a job so he can go to school, and he says the Democrats are with the communists. He don't listen to us."

I do not offer Louie as an example of the typical voter in 1984. Louie is Louie: one man, one vote; no more, no less. But I am convinced that the way he has reacted to what he saw on the TV debates represents something important in the politics of 1984 -- and especially beyond.

For you hear similar concerns on Wall Street from strong Reagan supporters who, unlike Louie, will still vote for Reagan Nov. 6. Their doubts are about Reagan in a second term. Jim Balog, for instance. He's an investment banker, widely respected as the senior executive vice president of Drexel Burnham Lambert Inc.

"The first four years have been pretty good four years," he was saying, during a long conversation about the economic outlook after the election.

"I am concerned that the residuals from the debates are that, God, we're going to face four more years with what looked to me to be a guy that might not be up to it physically and mentally. Now I'll still vote for Ronald Reagan, but probably with a little more residual concern there. The effect of those two debates was, well, first of all that Mondale did a good job in enunciating the facts. He seemed to me be in better command of the details of government than did Ronald Reagan. Being a Reagan fan at the beginning of the debates and being a Reagan fan after the debates, well, after the debates I was a Reagan fan with a little bit of reservation and concern: that maybe my man was a little tired, that maybe my man better be damned sure he has good advisers around him in a second term. I'm not as comfortable with the thought of Ronald Reagan's next four years as I had been. It left me with that lingering doubt.

"Before the debates I thought that Reagan would win the election and I thought that I was supercomfortable with that prospect. After the debates I have some nagging concerns that weren't there before. That's the difference. I'm just as committed. I think the country needs four more years of policies something like his and I don't want a reversal in that direction, but there's a nagging doubt about total ability and competence and vigor and that kind of stuff. I don't think I'm unusual in that feeling. Basically, he left that residue even with the cowboys among my peer group. They say, 'Yeah, he didn't look so good, but after all he's 73 years old.' Or: 'After all, you don't run the country on television.' Or: 'His experience and background will be what's really important when it comes to pushing the button, not what you see in front of a camera,' et cetera, et cetera. Lots of excuse-making, and they're excuses because they saw a flaw. They themselves saw something that concerned them."

When workers like Louie and Wall Street leaders like Balog express similar concerns, I'd say something's happening. To borrow Arthur Miller's memorable phrase from "Death of a Salesman," attention should be paid.