The clock is ticking on the Senate Foreign Relations Committee as its members brace for their last battle this week over the nomination of John R. Bolton (search) as U.S. ambassador to the United Nations.

But now a second Senate committee has been brought into the quest for information about Bolton's temperamental behavior. Late Tuesday, the top Democrat and Republican on the Senate Intelligence Committee huddled with Gen. Michael Hayden (search), deputy director of national intelligence, for a lengthy briefing about the 10 occasions over the last four years when Bolton requested the names of U.S. officials redacted in surveillance transcripts circulated by the National Security Agency (search).

Other officials made 400 such requests in the same time frame and the Intelligence Committee is reviewing the evidence because of its jurisdiction over NSA. Democrats hope to find evidence to support their claims that Bolton manipulated intelligence to suit his views even as they concede they have no idea whether the NSA data will prove relevant to those charges.

Meanwhile, talks continue about the State Department providing more documents to Congress.

"I think we're looking to see if there's other ways that we might be able to — or anything else we might be able to do that would help the committee in making its decisions," said State Department spokesman Tom H. Casey, referring to the Foreign Relations Committee, which is set to vote on Bolton's nomination on Thursday after five hours of closing arguments.

As that deadline draws near, the Intelligence Committee is quietly expanding its role in the Bolton nomination. In addition to the NSA briefing, FOX News has learned the panel is interviewing several individuals, including Fred Fleitz (search), the CIA officer who served as Bolton's chief of staff at the State Department. The individuals already have been interviewed twice by the Foreign Relations Committee.

In the unlikely event that the additional investigation produces new and damaging information about Bolton, it could potentially affect the timing or outcome of the panel vote on Thursday.

Late Tuesday came a sign of the increasing effectiveness with which Bolton's supporters have begun to mobilize: four letters to the Foreign Relations Committee signed by 150 of Bolton's former colleagues from three government agencies and a think tank. The letters include career and political appointees, mostly Republicans but also some Democrats decrying what they call "character assassination masquerading as a confirmation hearing."

•Click here to read the American Enterprise Institute’s letter (pdf)
•Click here to read the U.S. Agency for International Development’s letter (pdf)
•Click here to read the U.S. Department of Justice’s letter (pdf)
•Click here to read the U.S. Department of State/International Organization Affairs’ letter (pdf)

Senate Democrats met privately Tuesday to assess their chances of forcing another delay in the vote while their leader refused to rule out a filibuster.

"I don't think there's any way of determining that. The committee hasn't acted on that. That's going to happen Thursday or maybe even after that," said Senate Democratic leader Harry Reid of Nevada.

The committee Republicans who expressed reservations earlier also may be falling in line. Sen. Lincoln Chafee of Rhode Island told The Associated Press on Tuesday he reluctantly would support Bolton during the Senate vote.

I won't deny a lot of the information certainly brings great pause, but I fight the administration on so many issues; this is one of those that I've been with them on — to appoint their team," Chafee said.

Sen. Chuck Hagel of Nebraska, who has wavered, said he has not yet seen any evidence that would keep him from supporting the nominee but planned to make his decision at Thursday's committee meeting. Sen. George Voinovich of Ohio said he was reserving judgment but has been lobbied intensely by the Bush administration.

Secretary of State Condoleezza Rice has called for speedy confirmation of Bolton, arguing the delay thus far has set back the cause of U.N. reform. But in a letter obtained by FOX News that one of her aides sent to Sen. Barbara Boxer of California, Rice denied a Washington Post story claiming Rice ordered aides at a staff meeting to withhold evidence damaging to the nominee.

"I was in attendance at that meeting," wrote Matthew Reynolds, acting assistant secretary of state. "The secretary made no such statement. Rather, she simply made it clear to those present that no one should speculate to the press regarding what she did or did not know in relation to Mr. Bolton's nomination."

Click in the box near the top of the story to watch a report by FOX News' James Rosen.