Is a strong labor movement a good or bad thing for the nation's cities, and particulary the millions of city residents living near or below the poverty line?

The popular wisdom has always been "yes," that unions, cities and poor people have practically synonymous interests.

Most unions, after all, got their start in big cities. They forced up abysmally low wages paid by employers, assuring a dramatically improved standard of living for city residents. Unions have fought for minimum wages, for lower interest rates, for a rapid expansion of the national economy that should aid cities and poor folk everywhere.

Across the nation, however, several issues are now crystallizing that raise serious questions about private sector unions and cities. The questions may be just as serious as the much publicized problem of high wages and pension benefits won by municipal employee unions.

The gut question now is whether central cities, locked in tight competition with their suburbs for jobs and development, can survive as viable political economies - especially for the large share of the nation's minorities, poor, often unskilled and unemployed people who live within their borders.

Many of the problems of older cities have little to do with unions: outmoded industrial plants, lack of land for new plants, industries that unconscionably desert the city that gave them their start.

But other city problems are union related. The fact is that unions, by their very success, now represent an increasingly middle class group of workers -many of whom live in suburbs. And that's the constituency unions must represent - not the unorganized and chronically unemployed, the poor, the very young or old, the demoralized element that so often ends up loving on welfare and estranged from society.

Recent congressional approval of a minimim wage boost - from the present $2.30 an hour to $2.65 next year and $3.35 by 1981 - is a prime example of a union backed action that looks good for the city poor, but could turn out to be disastrous. The typical minimim wage worker, the AFL-CIO contends, is unskilled - young, black or female - and this group will benefit.

But the unions' real motive, critics say, is to bolster their own organizing and collective bargaining efforts. Most economists agree that the newest increase will cause teenagers to lose hundreds of thousands of jobs - at the very time cities are coping with the social dynamite of a 40 per cent black teenage unemployment rate. "It's absolutely insane," Brandels University economist Leonard when teenage employment is high and rising."

The minimum wage, says St. Louis urbanologist Norton Long, has come to be "a protective tariff against the cities, to store people off the labor market, so they won't complete with the unions. In fact it's taking away their bread card." To survive, he adds, cities must find employment for their vast reservoirs of unskilled minorities, undercutting the suburbs in labor costs. "But us bubble headed liberals," he says, "are opposed to doing that because we think it's degrading and demeaning."

Even if one supports minimum wages in general, it's hard to comprehend Congress's refusal to back a proposed 15 per cent lowering of the minimum wage for teenagers during their first six months on entry level jobs. The unions said the provision would cause employers to fire older workers and employ teenagers in their stead. But experience in Europe - where virtually all countries have subminimum wages for teenagers - shows no harm to adult employment.

Construction trade unions pose a dire threat to neighborhood and city renewal across the country through the 45 year old Davis Bacon Act. The law requires paying so-called "prevailing wages" - in practice ultra-high union construction wages, generally 50 to 100 per cent above what non-union contractors pay - on virtually any job paid for or subsidized by the federal government.

Davis Bacon was originally motivated in part by racial prejudice - fear that out-of-stage contractors, using inexpensive black labor, would undercut local builders. Today, by effectively prohibiting wage competition, Davis Bacon costs the taxpayers $2 to $6 billion annually in inflated construction costs o highways, military and public works projects. Yet to the AFL-CXIO Davis Bacon is a sacred cow; any urban Democrat would think long and hard before questioning it.

When city, neighborhood or non-profit groups try to use any form of dederal manpower of interest-rate subsidy to rehabilitate poor people's housing, they run head on into Davis Bacon and often find there's no way to build the project economically. The situation is particulary tragic when an attempt is made to use minority contractor (almost all non-union) or residents' own "sweat equity" in rehabilitation. That approach lets poor people develop valuable job skills while fixing up their own housing. But unions can, and often do, blow the whistle on such projects. The Secretary of Labor can grant waivers, but rarely does.

"I hate to come off anti-union, but boy, it makes me mad," says Philip St. George of the Urban Homestead Assistance Board in New York. "They're destroying neighborhoods."

Sometimes unions let housing projects proceed, especially if their own members are used to train minority apprentices. Neighborhood groups, unions and the city government have begun a model effort along those lines in Allentown, Pa. Yet the unions - or thoughtless bureaucrats - can stymie federally subsidized projects at will, simply by invoking Davis Bacon.

There are other points of neighborhood union contention. Construction unions, for instance, support freeways that decimate city neighborhoods. Transit unions can stop federally subsidized transportation projects - jitneys, mini-buses, possibly even deregulation of taxis to create better service for low income areas if they can show their jobs might, however remotely, be endangered.

Milton Kotler, executive director of the National Association of Neighborhoods, believes such impasses cn be overcome through local neighborhood union task forces. But as long as the unions, through their musle in Washington, hold a trump card on so many fronts, poor city neighborhoods and their residents will be at a permanent - and perhaps fatal - disadvantage.