Washington -- The escalating Senate fight over President Obama's cabinet appointments moved closer to a showdown on Thursday as two nominees headed to the floor with no Republican support.
Gina McCarthy, Mr. Obama's choice to lead the Environmental Protection Agency, and Thomas E. Perez, the nominee to be secretary of labor, were approved in committee with only Democratic votes and now face likely Republican filibusters.
The threat of further Republican attempts to thwart the president's ability to assemble his second-term cabinet has increased the likelihood of a fight over the Senate's rules, which allow the minority party to insist on a 60-vote threshold for almost every Senate action.
Democrats say that Senator Harry Reid, the majority leader, in recent days has been trying to gauge whether there is sufficient support among Democrats to force a rule change that would limit the filibuster on presidential nominees. He could conceivably try to enact a rule change with a simple majority – a tactic known as "the nuclear option." Any revisions to Senate rules usually require 67 votes, a threshold that is impossible to obtain without significant Republican support.
Republicans insist they are only standing in the way of nominees who merit more scrutiny and pointed to the advancement of two more Obama administration choices on Thursday: Sri Srinivassan, whose unanimous approval by the Senate Judiciary Committee sends him to the full Senate for confirmation to the United States Court of Appeals for the District of Columbia, and Ernest J. Moniz, the president's pick for secretary of energy, who was confirmed on a 97-to-0 vote by the full Senate on Thursday afternoon.
But many Democrats have become increasingly exasperated by routine efforts to stall and block presidential nominees. And they are now more supportive than ever of exploiting a technicality of Senate rules that would allow them to make changes with a simple majority of 51 votes.
"The showdown is coming," said Senator Jeff Merkley, Democrat of Oregon, who has been working with Senator Tom Udall of New Mexico on efforts to overhaul filibuster rules.
"And the leadership is very engaged in preparing how to deal with this and how to change this so advise and consent does not become an instrument of destruction."
To implement a rule change with a simple majority, Democrats like Mr. Merkley and Mr. Reid would have to overcome deep skepticism from many within their own ranks, particularly more senior senators who worry about the precedent such a move would set.
"You think you've got gridlock now?" said Senator Carl Levin, Democrat of Michigan. "You think you've got problems now? You will have a huge, huge outpouring of real anger."
"That means the next Senate," he added, "if the Republicans control it, you can expect them by majority vote to put through any rules change they want."
But the level of frustration among Democrats now has pushed many of them, including Mr. Reid, to believe that the situation has deteriorated so badly that it can be fixed only by doing something they once would have never considered.
The level to which the squabbling over nominees has sunk was vividly evident on Thursday as the Environment and Public Works Committee voted to send Ms. McCarthy's nomination to the full Senate. Republicans on the committee boycotted a vote scheduled for last week, leaving Democrats without a quorum.
That left Senator Barbara Boxer, Democrat of California and the committee chairwoman, with no other way to obtain a quorum but to round up Senator Frank R. Lautenberg, the ailing New Jersey Democrat who has been largely absent from the Senate in recent weeks.
Mr. Lautenberg appeared on Thursday to cast his vote, as did Republicans. All eight of them voted no, and Ms. McCarthy's nomination was reported out of committee by a 10-to-8 party-line vote.
Republicans said they could not vote yes because they believe that Ms. McCarthy and the E.P.A. have not been sufficiently transparent, including what they said was a failure to adequately answer almost 1,100 questions that they submitted.
"We're asking for openness and transparency as required by law," said Senator David Vitter of Louisiana, the committee's ranking Republican.
During the vote for Mr. Perez, who was reported out of the Committee on Health, Education, Labor and Pensions with a 12-to-10 vote, Senator Tom Harkin, the chairman, said that Republican efforts to block the nomination were not based on any legitimate concerns. "That was just delay for delay's sake," he said.
But Republicans said they believed that the involvement of Mr. Perez, who is head of the Justice Department's civil rights division, in a fair housing case in Minneapolis led to an unusual quid pro quo in which Justice agreed to stay out of another matter. And they have also pointed to what they said was his indifference in a racially charged case involving the New Black Panther Party, which conservatives have long complained was allowed to intimidate white voters in 2008.
"I understand that elections have consequences and presidents are entitled to be able to have cabinet members nominated and considered by the Senate within reasonable period of time," said Senator Lamar Alexander, the ranking Republican on the committee. "However, the Senate has a duty of advice and consent, and we would not be fulfilling our constitutional duties if we rushed to vote on Mr. Perez."
Most nominees to the post of labor secretary and E.P.A. administrator have breezed through their Senate confirmations in recent times. Some even won approval on a voice vote because there was so little resistance. That group includes, some Democrats have begun pointing out, former Labor Secretary Elaine Chao, the wife of Senator Mitch McConnell, the Republican leader.
This article originally appeared in The New York Times.