Sen. David Vitter, R-La., on Tuesday dropped his threat to filibuster the nomination of Gina McCarthy, President Obama's pick to lead the Environmental Protection Agency, potentially easing the path to confirmation for the woman expected to spearhead Obama's second-term crackdown on carbon emissions.

"I've had very productive conversations with EPA over the last several weeks and believe the agency has taken significant steps forward on our five transparency requests," Vitter said Tuesday. "These are huge, significant steps forward to bringing transparency to the agency, and I see no further reason to block Gina McCarthy's nomination, and I'll support moving to an up-or-down vote on her nomination."

Vitter had been among the most vocal critics of McCarthy, accusing the EPA of stonewalling requests for more information about its programs and McCarthy's history in the agency's clean-air division. Though Vitter - and the rest of the GOP contingent in the upper chamber - remains opposed to McCarthy ideologically, his decision could clear the way for an up-or-down vote as early as next week.

Obama recently called for his EPA to develop new carbon-emission standards on current and future power plants, sidestepping congressional Republicans who have shown little appetite to pass climate-change legislation. In doing so, McCarthy, if confirmed, instantly becomes one of the most powerful figures in the president's second-term cabinet.

However, other Republicans were not convinced to green light a vote on McCarthy. Sen. Roy Blunt, R-Mo., said Tuesday he would not remove his hold on the McCarthy nomination, citing unanswered questions about a state project being delayed by federal environmental officials.

"I am not asking the federal government to spend a dime, or for the agencies to green light the project's construction," he said in a letter released on Tuesday. "All I've asked is for three government agencies to agree on a simple set of facts."

He added, "As the saying goes, you're entitled to your own opinion, but you're not entitled to your own facts. The government needs to stop arguing with the government."

Added Emily Lawrimore, a spokesman for Sen. John Barrasso, R-Wyo., another major critic of McCarthy's: "His concerns remain the same and he will not support her nomination."

With McCarthy stuck in confirmation limbo, Senate Democrats have weighed changing the upper chamber's rule so the EPA nominee, among other Obama appointments, could be approved by a simple majority vote. They have not yet committed to such a move.

The White House did not have an immediate response to Vitter's announcement.

At the core of Obama's environmental push are tighter rules for power plants, more energy-efficient buildings and appliances, and additional green energy development on public lands. For environmentalists, stricter restrictions on current coal-burning power plants have long been viewed as the crown jewel of their agenda, but it's also an objective likely to prompt a flurry of lawsuits and stoke GOP charges of presidential overreach.

Obama has called for finalized greenhouse-gas standards for current power plants by June 2015.