In a letter sent to more than a dozen Republican senators Monday, NMB Chairwoman Elizabeth Dougherty said the process by which the proposal was drafted by her two colleagues is "flawed," and she questioned whether the agency had the authority to make such a rule change.

The NMB sent a proposed change to the Federal Register on Thursday that would lower the bar for unionizing at airlines and railroad companies. It is expected to be published in the Federal Register Monday or Tuesday and be subject to a 60-day comment period before it could be implemented.

Organized labor has been pushing for the change, but U.S. airlines oppose it -- including Delta Air Lines Inc. and Continental Airlines Inc., which are both preparing for large unionization votes.

That sets the stage for a battle that could spill into Congress or the courts. As such, the proposal also is shaping up as a litmus test on whether recent Obama administration appointees at key regulatory agencies will be able to shift from Bush-era labor policies to create a friendlier environment for unions.

Earlier this year, the White House named Linda Puchala, a former leader of a flight-attendant union, to the mediation board, succeeding Read Van de Water, a former lobbyist for Northwest Airlines. Two of the three board members are now former labor leaders.

Ms. Dougherty is a registered Republican who served as a labor adviser to former President George W. Bush earlier this decade.

The NMB oversees labor relations at companies -- mostly airlines and railroads -- that are governed by the Railway Labor Act of 1926.

Under a 75-year-old interpretation of the Act, any employees who don't vote on whether to create a union are counted as "no" votes. That means a union can't be approved without a full majority of all employees voting in favor of it.

Most other companies fall under the National Labor Relations Act. Under that law, a union can be created as long as a majority of all votes cast are in favor of collective bargaining. In such elections, nonvotes don't count either way.