The Senate blocked President Obama's nominee for the National Labor Relations Board on Tuesday, as Democrats fell short of the 60 votes needed to break a Republican-led filibuster.

Two Democrats, Sen. Blanche Lincoln of Arkansas and Ben Nelson of Nebraska, broke with their party to vote against Craig Becker in the key procedural vote, which failed 52-33.

The vote also was significant for displaying the GOP's newfound muscle, as it came days after Republican Scott Brown of Massachusetts was sworn in as the newest senator, breaking the Democrats' 60-vote supermajority -- though the measure would have failed even without Brown's help, given Lincoln's and Nelson's no votes.

Republicans have held up Becker's confirmation for months. They say Becker would push an aggressive union agenda at the agency that referees labor disputes between unions and management.

Brown said Tuesday he couldn't vote in favor of Becker. 

"My first priority in coming to Washington is to create jobs and put people back to work," he said in a written statement. "Craig Becker's theories about how the workplace should function, if ever put into practice, would impose new burdens on employers, hurt job creation and slow down the recovery." 

On Monday night, Nelson had announced he would join the filibuster, a rare move for the conservative Nebraska Democrat, who is fond of saying that having been a governor responsible for choosing his own nominees, he gives great leeway toward a president's choices.

Only in "extraordinary circumstances" does Nelson join a filibuster, he said, a defining phrase that arose from a 2005 judicial nominations impasse.

Many Republicans say they believe Becker has his own agenda when it comes to so-called "card check" legislation, a proposal that would make it easier for workers to unionize. Nelson said he agrees.

This legislation has been stalled since it was first proposed. Sen Tom Harkin, D-Iowa, chairman of the Health, Education, Labor, and Pensions Committee, is the point man for finding a compromise, working with Democratic Sens. Mark Pryor of Arkansas, Arlen Specter of Pennsylvnia and others.

Nelson has said he is opposed to the Employee Free Choice Act, the formal name for the bill.

"Mr. Becker's previous statements strongly indicate that he would take an aggressive personal agenda to the NLRB, and that he would pursue a personal agenda there, rather than that of the administration," Nelson said. 

"In addition, the nominee's statements fly in the face of Nebraska's Right to Work laws, which have been credited in part with our excellent business climate that has attracted employers and many good jobs to Nebraska," he continued. "Considering these matters, I will oppose the upcoming cloture motion and the nomination."

The five-member National Labor Relations Board is responsible for deciding cases under the National Labor Relations Act.

Fox News' Trish Turner contributed to this report.