WASHINGTON (AP) — Senate Republicans said Thursday they would not support five nominees to the National Labor Relations Board, raising the possibility the troubled agency could be rendered mostly inoperable later this year.

The board has already been working under a cloud since January, when a federal appeals court said the president violated the Constitution by filling vacancies on the board through recess appointments without Senate confirmation.

But an impasse over the latest slate of nominees could pose broader problems. The five-member board needs at least three sitting members to conduct business. It has a bare minimum of three now, but the term of its chairman, Mark Pearce, expires in August.

At a Senate confirmation hearing for the nominees, GOP lawmakers renewed their claim that the board has no legitimacy to act while the two recess appointees — Sharon Block and Richard Griffin — are serving. Republicans expressed further annoyance that President Barack Obama is including Block and Griffin in his slate of five nominees for full terms on the board.

"The president could nominate two equally qualified members who did not sit on the NLRB when a court had decided they were unconstitutionally there," said Tennessee Sen. Lamar Alexander, senior Republican on the Senate Health, Education, Labor and Pensions Committee.

Alexander said the White House has shown "a troubling lack of respect for the constitutional separation of powers and for the Senate's role of advice and consent that is standing in the way of this confirmation process."

Under questioning, Griffin and Block both said they decided to remain on the board because they took an oath to serve. They said it was proper to continue serving on the board until the Supreme Court steps in, because appeals courts have issued contradictory rulings about the right of a president to make recess appointments.

Democrats on the panel accused Republicans of obstructionism because the GOP and its allies in the business community have been unhappy with some of the union-friendly decisions issued by the board during Obama's administration.

Sen. Elizabeth Warren, D-Mass., accused Republicans of a demonstrating a pattern of delaying or blocking confirmation of appointments to the Environmental Protection Agency, Labor Department and Consumer Financial Protection Bureau.

"This is about complete obstructionism because the minority senators don't like the agencies, and they don't like the work these agencies do," Warren said.

The Obama administration has insisted it wants Griffin and Block confirmed as part of the five-member package of nominees. Obama also has renominated the board's chairman, Pearce, to another full term. All three are Democrats. The other two nominees, lawyers Harry I. Johnson III and Philip A. Miscimarra, are Republicans.

While the committee's Democratic majority was expected to endorse the nominees, Republicans were likely to demand 60 votes for a full vote to proceed on the Senate floor.

The prospect of a crippled NLRB has angered labor unions, which count on the agency to resolve worker complaints of unfair labor violations and issue key legal decisions on labor-management disputes.