Du Pont Co. workers overwhelmingly rejected an organizing bid by the United Steelworkers union in voting that ended last night--stopping the union at all 14 plants where it attempted to set up shop.

The defeat wrecked a eight-year effort by the union to gain a foothold in the nation's largest chemical manufacturing firm. It also put a large dent in organized labor's organizing ability.

The Steelworkers' drive began as an attempt to put 65 plants in the Du Pont empire into a single collective bargaining unit. But, through a series of events and tactical changes, the union recently shifted its energies to setting up individual bargaining units at 14 of Du Pont's 100 domestic facilities.

Last night's crushing defeat--in which the Steelworkers also lost badly to six independent unions favored by the company--was hailed by Du Pont officials as proof that the company's strategy of dealing with its workers at the local level is superior to bargaining with a national union.

"We are delighted that employes at all 14 Du Pont plants rejected an eight-year organizing bid by the Steelworkers union," said Carl De Martino, Du Pont's vice president of employee relations.

"We view the result as a vote of confidence in our treatment of our employes, and we recognize the obligation that this entails. We will continue to deal successfully with employe relations at the local level--a principle we firmly believe is in the best interest of our employees," De Martino said.

There was no immediate reaction from union officials. But, earlier yesterday, Steelworkers spokesman Russell W. Gibbons said "it's not the end of the world if we don't knock them off. That company probably has put more money into trying to keep us out than they could have spent on three union contracts by now," he said.

A Du Pont spokesman also downplayed the significance of the election before the results were known: "Even if they win all 14 plants, that won't mean much. That will be only a small portion of the company."

A spokesman for the AFL-CIO, of which the Steelworkers union is a prominent member, put it this way: "If the Steelworkers win, we'll call it a major victory for the U.S. labor movement. If they lose, we'll say it's not a landmark vote."