“Gina McCarthy deserves a vote,” said Ms. Boxer, visibly angry. “I have delayed a vote for three weeks. I was assured by Senator Vitter that once he received answers to 1,000 questions — a record-breaking number — they would allow us to move forward with the vote.”

Ms. Boxer and other committee Democrats noted that Republican members had submitted 1,079 questions to Ms. McCarthy, compared with 157 total questions for Mr. Obama’s first E.P.A. administrator, Lisa P. Jackson, and 305 for Michael O. Leavitt, who served in the post under President George W. Bush.

Democrats boycotted a vote on Mr. Leavitt’s nomination in October 2003, saying he had failed to adequately answer their questions on environmental policy. The move forced a two-week delay in his confirmation.

Mr. Vitter, in a letter to Ms. Boxer and at a news conference Thursday, said that Republicans were not seeking to obstruct the nomination or harass the nominee.

“These requests are all about openness and transparency and things required by law,” Mr. Vitter told reporters. “We’re not asking the Obama administration to walk away from their views on carbon or anything else.”

He said that the E.P.A. had been unresponsive to Freedom of Information Act requests, that it had failed to provide scientific data supporting regulations and that top officials had used aliases in e-mail addresses, a practice now under review by the agency’s inspector general.

Ms. Boxer said she intended to press for a vote on Ms. McCarthy, which she can do if all 10 Democratic members of the committee appear in person. Senator Max Baucus of Montana was at another hearing, and did not attend Thursday’s meeting, nor did Senator Frank R. Lautenberg of New Jersey, who has been largely absent from the Senate in recent weeks because of health concerns.

“It was not my intention to vote this out with just Democrats,” Ms. Boxer said. “I am asking my Republican colleagues to come home, come back to your responsibility. If you want to be here and vote no, be here and vote no. That is your prerogative. But vote.”