Forty-five years ago Adolph Coors Jr., catering to the post-Prohibition thirst of union members, invited a union to organize workers at the brewery founded by his immigrant father during the 1870s in the Rocky Mountain foothills west of Denver.

Now a third generation of Coors, brothers Joseph and William, have defied the labor movement in winning a decertification election last month to oust the brewery workers union that their father had welcomed.

A union-led boycott of Coors, the nation's fifth largest supplier of beer, continues despite a sales market limited largely to Western states.

But the only visible reminder of the 20-month strike that toppled Brewery Workers Local 366, an independent affiliate of the AFL-CIO, are a couple of leftover picket signs propped up outside the main gate of the massive gray concrete beer-making complex, reputed to be the world's largest brewery.

"I think we'll be back," perhaps with the backing of a large international union, says Local 366 President James B. Silverthorn, whose union is now bereft of dues-paying members. The local lives off donations from other unions as it continues the boycott and contemplates its future.

Coors officials speak of the union in the past tense and vow to outlast the boycott, although they concede it has contributed to the company's drop in barrel-volume sales, which are down about one million from 13.5 million barrels in 1976, reversing a normal 10 percent annual sales gain.

The Coors union fight has attracted an unusual amount of attention because of the boycott, the company's controversial use of lie detectors in screening new employes, and the Coors brothers' outspoken political conservatism. But the workers' vote here to kick out the union reflects an increase in union decertification votes across the country in recent years.

In 1967 the National Labor Relations Board conducted 234 decertification elections. Unions lost 165. In 1977, 849 decertification elections were held, and unions lost 645.

The reasons vary, but often increasing corporate militancy and sophistication is key to outmaneuvering unions.

At Coors, union officials accuse the company of scaring workers into believing they would lose their high-paying jobs if the strikers came back to work. Company officials say that the union misread its members' desires and came to be seen as an obstacle to good labor-management relations.

Both sides may be partly right. "I earned my job and I wasn't going to give it to some striker," said a young man who was hired as a strike replacement. Said another recently hired Coors worker: "The union was dead and didn't even know it." Neither wanted to be quoted by name. Both said they voted against the union.

Silverthorn conceded that most union members had already voted with their feet by returning to work before the decertification vote was held Dec. 13 and 14. "The average pay is more than $8 an hour," he said. "You can't find that kind of money anywhere else here. Sometimes money isn't everything, but it's hard to tell people that."

When the strike started in April 1977, most of the brewery's 1,500 production workers walked out. By the time of the vote, according to a union count, all but 500 had gone back to work and the rest had been replaced.

The vote was 993 to 408 to decertify the union as bargaining agent for the brewery workers. The strikers were not allowed to vote, under NLRB rules, but even if they all had cast ballots for the union, it would not have changed the outcome.

The union appeared to be firmly entrenched as recently as 1976 when the company, invoking Colorado's Labor Peace Law, called for a vote among the brewery workers on whether the union could bargain to retain its long-standing union shop rule. The union shop clause won with the support of more than 92 percent of the Coors workers.

But negotiations for a new contract bogged down over issues such as the company's demand for modification of seniority rights. The union went out on strike on April 5, 1977, triggering a national union boycott of Coors beer.

It was then that the company insisted on revocation of the union shop rule, which, along with reinstatement of the strikers, became the overriding strike issues.

"We didn't believe non-strikers should be forced to join the union or that people should be forced to pay union dues to support the boycott," said Richard Kellogg, director of labor-management relations for Coors. Union leader Silverthorn said the company went so far as to type out union resignation letters for returning employes.

Then, early last year, the company asked for an election on whether to retain the union. After the company agreed to a $254,000 back pay award to resolve an arbitration dispute that held up the election, the campaign was on.

According to the union, company supervisors wearing buttons saying "Vote No -- Ask Me Why," warned that the union's demand for reinstatement of up to 500 strikers could result in the layoff of 500 workers.

A company poster depicted a cigarsmoking AFL-CIO President George Meany riding a horse labeled "Local 366" and dragging a forlorn figure labeled "Employe" through a thicket of cactus labeled "Strike... Boycott... Smear Tactics."

"They got the fear instilled in them," said Silverthorn.

Said Kellogg: "If the union truly believed there was intimidation, they should have filed an unfair labor practice charge." Said Silverthorn: "They stayed right on the edge of the law."

Kellogg maintains that the union was ambushed by the tactics of its own overzealous supporters. While conceding that the strike was "generally nonviolent," he said some workers' cars were vandalized. "The union to them was the group that slashed their tires," he said.

The union also made a point of what boycott supporters were to characterize as human rights issues. These issues included minority hiring and promotion practices, the use of lie detector tests and the company's broad authority, sanctioned over the years in the union contract, to dismiss workers for reasons ranging from violating local morals to making derogatory remarks about Coors beer.

Lie detector tests routinely given prospective employes covered such matters as whether a person had overdue debts, used marijuana, discussed company business with unauthorized persons or "had anything in your personal life that might tend to discredit or embarrass this company if it were known." Now the test has been shortened and modified, but still asks about "subversive, revolutionary or communist activities" and whether the applicant is seeking to join the company "so you can do it or any of its employes harm."

The Coors brothers were unavailable for interviews last week, but in previous interviews have made it clear they don't think much of unions, although they deny they are anti-union.

Joseph Coors, president of the company and a contributor to a number of conservative causes including the National Right to Work Committee, the Committee for Survival of a Free Congress, and the John Birch Society, has said he sees little if any need for unions.

William Coors, chairman of the board, hailed the decertification election as a "declaration of independence," meaning that for the first time since 1934 the company can now "deal directly" with workers without union involvement.

A new corporate advertising campaign, aimed at countering allegations made by boycott leaders, carries out this people-to-people theme, showing smiling black, Hispanic and female workers under the slogan: "At Coors. People Make the Difference."