Four of President Barack Obama's nominees moved forward Thursday with one, MIT physics professor Ernest Moniz, being unanimously confirmed by the Senate to be secretary of energy.

Another nominee -- Sri Srinivasan to serve on the U.S. Court of Appeals for the District of Columbia Circuit -- was OK'd by the Senate Judiciary Committee and seems certain of confirmation by the full Senate.

But two other nominees -- Thomas Perez to head the Department of Labor and Gina McCarthy to lead the Environmental Protection Agency -- face an uncertain future on the Senate floor.

In a party-line vote, a Senate committee Thursday approved the Perez nomination. 

But Sen. Charles Grassley, R-Iowa, strongly opposes Perez, who is now the assistant attorney general for civil rights.

The Iowa Republican accuses Perez of improperly arranging a swap. If the city of St. Paul, Minn., would withdraw a major fair housing case which was about to be argued before the Supreme Court, then the Justice Department would agree to not go to court in support of a whistleblower suing the city.

In his confirmation hearing last month before the Senate Committee on Health, Education, Labor, and Pensions, Perez said a career Justice Department attorney had decided that the whistleblower had a weak case that didn’t merit intervention by the Justice Department on his side.

Led by Rep. Darrell Issa, R-Calif., House Republicans have also crusaded against Perez. Issa got into an angry confrontation with Attorney General Eric Holder over the Perez nomination at a House Judiciary Committee hearing Wednesday.

Another key Obama nominee, McCarthy, won approval Thursday on a party line vote by the Senate Environment and Public Works Committee.

Last week Republicans on the panel boycotted a meeting of the committee to underscore their demand for greater openness from the EPA on how it reaches its decisions and how it strikes deals with environmental groups to settle lawsuits.

Ranking Republican member, Sen. David Vitter, R-La., said Thursday Republicans were now willing to move ahead on McCarthy because “we’re finally making real progress on the five key transparency requests that have been the focus of all the Republican members’ concerns about this nomination process.”

Vitter said the EPA had agreed to give GOP senators significantly more information on how it reaches its decisions. He said if the EPA provides more transparency, he would support handling the nomination on the Senate floor without a cloture vote, which would require 60 votes. If all of the Republican’ request for EPA transparency in five areas are met, Vitter said he would vote for McCarthy’s nomination on the Senate floor.

But referring to the Republican opposition, committee chairman Sen. Barbara Boxer, D- Calif., said, “I have never seen a nomination handled this way ... . I’m stunned at this. It’s kind of holding somebody hostage until you get an answer you want to have.”

Even if Vitter relents, the McCarthy nomination still faces a hold from Sen. Roy Blunt, R-Mo., who is protesting what he calls “bureaucratic infighting” among federal agencies which have delayed an environmental impact statement on the St. Johns Bayou-New Madrid Floodway Project in his state.

Obama got a significant victory Thursday when the Judiciary Committee unanimously approved his nomination of Deputy Solicitor General Srinivasan to serve on the Court of Appeals for the D.C. Circuit, the nation’s most powerful appeals court.

So far in his presidency Obama hasn’t gotten any nominee confirmed by the Senate to the D.C. circuit appeals court. That court is now divided between four judges appointed by Republican presidents and three judges appointed by Bill Clinton. (There are also six senior judges with a reduced workload who take part in some cases.) The court has four vacancies.

Judiciary Committee Chairman Sen. Patrick Leahy, D-Vt., has called that court “more important than the Supreme Court because on so many of the issues that go there, they will have the final word.” It’s the end of the road for most cases since the Supreme Court accepts only a fraction of the requests for appeals. The court hears most of the challenges to decisions of regulatory agencies such as the EPA.

This story was originally published on Thu May 16, 2013 10:22 AM EDT

Labor Secretary Hilda Solis will not return in President Barack Obama's second term, the White House confirmed Wednesday.

Solis, formerly a California congresswoman, said she made the decision to step down from her role over the holiday season.

Richard Vogel / AP file photo

Flanked by California Attorney General Kamala Harris, left, and Los Angeles County Sheriff Lee Baca, right, U.S. Secretary of Labor Hilda Solis talks during a news conference at a symposium on human trafficking on Friday Nov. 16,2012 in Los Angeles.

"Over the Christmas and New Year holidays with my family in California, I enjoyed my first opportunity in years to reflect on the past and my future, with an open mind and an open heart," she said in a statement. "After much discussion with family and close friends, I have decided to begin a new future, and return to the people and places I love and that have inspired and shaped my life."

Many Cabinet officials chose to stay on for only one presidential term, although a White House official confirms to NBC News that the heads of the Health and Human Services, Veterans Affairs and Justice Departments will remain in their current posts.

In a statement, Obama called Solis "a tireless champion for working families."

"Over the last four years, Secretary Solis has been a critical member of my economic team as we have worked to recover from the worst economic downturn since the Great Depression and strengthen the economy for the middle class," he said. "Her efforts have helped train workers for the jobs of the future, protect workers’ health and safety and put millions of Americans back to work."

The vacancy sets up the absence of yet another female member of Obama's cabinet. The administration has come under pressure to consider gender and racial diversity after recently appointing white men to top posts at the Defense, Treasury and State Departments.

The news was first reported by Buzzfeed.