The strike at Greyhound Bus Lines, in which employes are having to choose between loss of their jobs or cuts in pay, is a fitting symbol for the last few years of U.S. labor history: it has not been a good time for labor unions.

The bleak union outlook seems graphically summed up in the two-week-old spectacle of the dethroned bus drivers: hard-line managers operating the company despite the walkout, the newly competitive landscape of deregulation, a pool of unemployed replacement workers created by recession, automation and other developments.

It comes against a pattern that over the last several years has included concessions at the bargaining table, declines in the union share of the work force and weakened political influence at a time when many labor leaders see political solutions as their best hope.

Among other difficulties:

* Labor is finding it hard to deliver at the bargaining table. Major collective-bargaining agreements so far this year have provided for an average pay raise of 1.7 percent for the first contract year, compared with 9.1 percent the last time the same parties bargained.

About one-fifth of those affected took wage cuts, and another fifth got no specified wage increases. The average raises in this year's contracts are the lowest in the 15 years for which statistics have been kept.

Even concessions were not enough to prevent facilities from closing. According to a study by the Bureau of National Affairs, at least 27 companies or plants closed despite union givebacks in 1982.

Strikes do not work as they once did. Not only at Greyhound, but also at numerous other companies large and small, management is standing up to strikes and sometimes forcing them in order to break unions.

Unions also are contending with a loophole in the bankruptcy law that has allowed companies to get out of union contracts on grounds that they are financially burdensome.

* In 1980, only one of five U.S. workers carried a union card, compared with one of three in 1945.

While conceding that the membership totals have not kept pace with the growing work force, AFL-CIO officials argue that such figures are misleading and point out that the number, if not the percentage, of union members grew from 21.2 million in 1970 to 22.4 million in 1980.

Moreover, the AFL-CIO membership of 13.8 million, according to spokesman Murray Seeger, does not include those on layoff, sick leave or between jobs. He said also that the number of potential union members is far smaller than the "total work force" figure used by government statisticians.

The unemployed, the self-employed, supervisors, domestic and casual workers and most youngsters under 18 should not be counted, he said.

* Some of the labor movement's key industrial unions have been staggered by the biggest losses in membership. The United Auto Workers reports a decline from 1.5 million in the mid-1970s to 1.1 million. The United Steelworkers said it has lost 43 percent of its members, declining from 1.3 million in the late 1970s to 750,000.

* In 1981 the Gallup Poll reported public approval of labor unions at an "all-time low," although, it added, "positive attitudes outweigh negative by a 3-to-2 ratio." In 1936, the first poll on union sentiment registered 72 percent positive, while in 1981 55 percent approved of unions.

* The largest new block in the work force has been slow to become unionized. Although about 53 percent of women are working, only 19 percent of those in salaried positions have joined unions, compared with 31 percent of men.

Management and labor partisans as well as other observers attribute this state of affairs to an assortment of factors.

They include major economic upheavals, particularly in basic "smokestack" industries, long considered the backbone of organized labor; rapid technological change; managers' increased sophistication in labor relations; labor leaders' slowness to adapt and an anti-union climate established by the Reagan administration.

That climate began when the administration broke the Professional Air Traffic Controllers Organization, emboldening some employers to try the latest tactics in what is seen as union-busting.

"That had a significant psychological impact that is hard to quantify," said John Tysse of the U.S. Chamber of Commerce, referring to the administration action.

Still, many experts caution that, while organized labor is seriously ill, predictions of its death are premature.

"I don't see labor as a dying institution. That presupposes they won't change," said Randy Hale, an official of the National Association of Manufacturers, an adversary of labor organizations that recently gave its former Committee for a Union-Free Environment a less controversial name.

"We're in a volatile situation, and whether it moves in labor's favor or not is too early to tell," said Harley Shaiken, a labor analyst at the Massachusetts Institute of Technology. "The trouble is, some people have read the possibility as a certainty."

Some of the negative developments could have a positive side for labor, he added. For instance, while new technology has robbed unions of jobs, the resulting loss of job security could drive workers into the arms of union organizers if they seem able to offer protection.

The Communications Workers of America are among the pacesetters on this front. Public employe and service unions have been growing.

Many labor officials contend that their current turmoil is only periodic in a movement that has survived many tough times. But some, such as Robert A. Georgine, head of the AFL-CIO's building trades department, admit to occasional depression.

Anti-union forces, he said, "have gotten a hell of a lot stronger and more effective . . . . Sure, I get depressed at times when I feel that, no matter what we try, we end up in a blind alley." Labor leaders must make adjustments in some outmoded approaches, he said.

Besides its bold gamble in endorsing Democratic presidential candidate Walter F. Mondale, the AFL-CIO under President Lane Kirkland has begun other damage-control measures.

These include increased organizing efforts, especially in potentially fertile white-collar and high-technology areas and among women and minorities; joining business and public officials in seeking ways to deal with foreign competition and save U.S. industry and retooling antiquated communications and public-relations machinery.

In most of these areas, any results are expected to come at a glacial pace, if at all.

"I think this is the toughest time for labor since its legitimization back in the 1930s," Tysse said. "The public perception is that labor is a weak, and some say disappearing, institution . . . . The fundamental question is, what do I get for my dues? The answer just isn't the same as it used to be."

Some of labor's problems, specialists noted, stem from its success. For example, laws it helped to pass, such as those involving health and safety, pensions, fair-labor practices, civil rights and other protections, have placed the government in the role once filled by unions as guardian of workers' rights.

On the other hand, specialists noted, business forces have not accomplished much of their agenda for change either, for instance in key laws that they consider too favorable to labor. For either side, blocking legislation is much easier than getting it passed, the specialists said, so both sides have been limited largely to defensive victories.

Many specialists said they expected that changes in the labor-management relationship will be gradual and moderate and that the adversarial relationship will always exist to some extent.