In what could be a blow to the United Auto Workers and other unions, the Republican-majority NLRB said it would decide whether to curtail a union's ability to organize workers though a simplified process known as neutrality and card-check agreements.

The case probably won't be decided until sometime in 2005, and the outcome of this November's presidential election will have a major impact. The party that occupies the White House gets to control three of the five seats on the board, which regulates union-management relations in the United States.

The current 3-to-2 GOP majority would swing back to more union-friendly Democratic control if Sen. John Kerry of Massachusetts, the presumptive Democratic nominee, wins the presidency.

''This is starting a discussion on the most important issue in American labor law of the current period, certainly in the last 10 years,'' said John Raudabaugh, a partner at the Detroit law firm of Butzel Long and a former Republican member of the NLRB from 1990 through 1993.

Under neutrality and card-check agreements, companies agree to recognize a union that has collected signature cards from a majority of workers indicating they want to join, without forcing the union to go through a traditional secret-ballot election.

The UAW has used this simplified process in recent years to organize workers in the mostly nonunion auto parts industry, winning victories at Johnson Controls Inc., Magna International, Dana Corp., Metaldyne Corp. and other companies. Unions more and more are turning to this process because victory through the standard secret-ballot election can take years to achieve because of corporate legal challenges.

But in a case brought by an anti-union group known as the National Right to Work Legal Defense Foundation, the NLRB voted late Monday along party lines to examine how soon employees can challenge a company's recognition of a union as bargaining representative when a secret-ballot vote has not been taken.

The case involves the UAW's efforts to organize a Dana plant in Upper Sandusky, Ohio, and a Metaldyne plant in St. Marys, Pa. Both companies agreed to recognize the union after card-check campaigns in 2003. Within weeks, anti-union employees, supported by the Springfield, Va.-based Right to Work foundation, filed petitions with the NLRB to kick out, or decertify, the union. Rulings at the NLRB's regional level dismissed the petitions, saying a union and a company needed reasonable time to negotiate a contract.

By taking an appeal of the case this week, the Republican majority on the NLRB board in Washington, D.C., said it was not making any judgments on the merits. But its ruling raised the stakes for unions and employers.

The majority Republicans consists of NLRB Chairman Robert Battista -- a Detroit management labor lawyer from Butzel Long who once represented the Detroit Newspapers during the newspaper strike of the mid-1990s -- and members Peter Schaumber and Ronald Meisburg.

Dissenting Democratic members Wilma Liebman and Dennis Walsh said the right to voluntary agreements between unions and managements without a secret-ballot vote of the members was settled 40 years ago. It called the majority's willingness to take the case ''unsupported -- and, indeed, highly questionable.''

UAW President Ron Gettelfinger echoed that Tuesday.

''Card-check elections are a fair and efficient procedure, recognized for decades by the National Labor Relations Board and the courts of the United States. Their use should be expanded, not curtailed,'' he said in a statement.

Raudabaugh said a decision is likely to turn on the question of how soon a decertification vote can be held once a union has won an agreement with management. In normal cases anti-union workers have to wait a year or more to try to throw out a union. The NLRB majority could rule that a decertification vote could be held immediately after a union signs a neutrality agreement with a company.

Both sides claim to represent the spirit of workplace democracy. Anti-union activists contend that neutrality agreements and card-check campaigns can be manipulated by union and management so that rank-and-file workers never get to express an opinion.

Clarice K. Atherholt, an assembler at the Dana plant in Ohio, gathered signatures for the decertification effort. The failure to hold a secret ballot on union membership, she said, was ''the most unfair, undemocratic, un-American way that I have ever heard of.''

But Stewart Acuff, head of organizing for the AFL-CIO, the nation's leading union coalition, said the NLRB's traditional secret-ballot process has been hijacked by anti-union forces. He said companies routinely stall elections, fire union organizers and threaten to close plants in advance of a vote.

''It's a shame and a disgrace,'' he said. ''Workers should not have to exercise an extraordinary level of courage to express support for a union, but that's the way it is in the United States now.''