Republicans on Thursday boycotted a congressional hearing for President Obama's nominee to lead the Environmental Protection Agency, putting the nomination in doubt and enraging White House officials who dismissed the act as the most blatant case yet of GOP obstructionism.

The boycott was a major blow to Gina McCarthy's chances of winning a filibuster-proof confirmation vote in the Senate, given that, without the Republicans, progressives can't even get McCarthy through committee.

Republicans said they refused to move ahead on McCarthy until the EPA adequately responds to their questions about her, but more broadly the standoff has become a proxy fight over Obama's second-term environmental plans.

The Republican power play comes a day after GOP leaders delayed a separate committee vote on Thomas Perez, Obama's pick to lead the Labor Department.

"We call on Republicans in the Senate to stop gumming up the works when it comes to the confirmation process of nominees who are enormously qualified for the jobs," retorted White House Press Secretary Jay Carney.

Another senior administration official wasn't so polite.

"It's bull [expletive]," he said. "This is getting old. It has got to stop."

It's not just Obama's cabinet nominees who have met stiff Republican resistance on the Hill. Many of the president's judicial nominees have been either delayed or blocked by Senate Republicans -- it takes a nominee eight months on average to get confirmed.

Also on Thursday, Republican leaders announced they would not nominate candidates to Obama's Independent Advisory Board -- a key component of the president's health care reforms -- saying the unelected officials could deny health coverage to seniors.

The GOP's unyielding opposition to the president's nominees had congressional Democrats renewing calls for changes in filibuster rules, saying the Senate has been paralyzed by the partisan sniping.

"You know why some of us are going to be in favor of reforming the rules of the Senate?" Sen. Ben Cardin, D-Md, said. "It's because of abuses like this."

Republicans counter that they're merely following the Democrats' lead. In 2003, some of the same Democrats now decrying the GOP boycott did the same thing to Michael Leavitt, then-President George W. Bush's pick to run the EPA.

If confirmed to head the EPA, McCarthy could become one of the most influential members of the Obama cabinet. With climate-change legislation facing long odds on Capitol Hill, Obama could sidestep Congress and rely on the EPA to limit carbon emissions without congressional approval. Environmentalists are urging Obama to institute tougher rules for existing power plants.

"I really think it's just at this point, an unelected bureaucrat being able to define issues that involve hundreds of billions of dollars impacting the economy in a whole lot of ways -- it's just become an incredibly significant part of our American government, and any nominee should undergo scrutiny," Sen Jeff Sessions, R-Ala., said of McCarthy.

Regardless of its outcome, the McCarthy nomination could spawn an overhaul of Senate rules.

"This type of blanket, partisan obstruction used to be unheard of," said Senate Majority Leader Harry Reid, D-Nev. "Now it has become an unacceptable pattern. Republicans will use any procedural roadblock or stall tactic available to deny the president qualified nominees."