There are many bills moving to constrain the NSA's domestic datamining and surveillance programs, but the senior Senator from California is in control.

Drew Angerer / Getty Images Senator Dianne Feinstein talks to reporters on Capitol Hill, Sept. 17, 2013 in Washington.

The U.S. government may be closed down, but when it comes to reforming the National Security Agency’s domestic surveillance and data-mining programs, Congress has been busy.

In coming weeks members of the House and Senate will put out multiple bills seeking to constrain the NSA. There will be bills to outlaw the NSA’s bulk collection and data-mining of American’s telephone and internet records. There will be bills to reform the Foreign Intelligence Surveillance Act court, which allows the NSA and law enforcement to spy on Americans. There will be bills to establish third-party oversight of NSA programs that collect data from American citizens.

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For all that activity, though, only one member of Congress really matters in the fight: Sen. Dianne Feinstein, chairman of the Senate Select Committee on Intelligence, and a member of the Senate Judiciary Committee. And those who want dramatic changes aren’t particularly excited about it.

Feinstein is a centrist on national security matters, and she is seen by key staffers in both chambers as the determining factor in how many Democrats vote. “She’s the heavy hitter here,” says one Senate aide involved in the effort to constrain the NSA.

Feinstein made her presence felt this week when she came to the Senate Judiciary Hearing chaired by Senate Patrick Leahy of Vermont, a long-time opponent of parts of section 215 of the Patriot Act. Last summer, NSA contractor Edward Snowden revealed that section 215 has been used by the NSA to authorize the creation and maintenance of massive databases of Americans’ call and internet records. Leahy is preparing to unveil legislation soon that would stop the NSA’s program and limit the agency in other ways. Feinstein, arriving at Leahy’s hearing and asking for the floor even before the ranking Republican had a chance to speak, made it clear she was having none of it.

Feinstein said that 9/11 might have been prevented if the U.S. had had the bulk metadata collection program in the summer of 2001 when then-CIA chief George Tenet knew there was an imminent terrorist threat, but didn’t know how to uncover it. “I believe that if this were to happen again, with this program and other programs working in combination, we have an opportunity to pick that up. Absent these kinds of technological programs we do not have the opportunity to pick that up.” And in case it wasn’t clear where she stood on the continuation of the program of bulk collection of American’s metadata, she said, “I will do everything I can to prevent this program from being canceled out.”

Even though Leahy is likely to have the support of libertarian GOP Senator Mike Lee of Utah and House Judiciary Chair James Sensenbrenner, other Democratic Senators on the Judiciary committee are expected to follow Feinstein’s lead, including Rhode Island’s liberal Senator Sheldon Whitehouse, who is hawkish on national security and intelligence matters.

Leahy’s bill isn’t the only one Feinstein’s likely to derail. Senators Ron Wyden of Oregon and Mark Udall of Colorado, longtime opponents of the 215 bulk collection of Americans’ metadata, have a bill in the intelligence committee that would also ban the program.

Others seeking to constrain the NSA may have better luck collaborating with Feinstein rather than opposing her. Richard Blumenthal of Connecticut is expected to propose introducing an adversarial element to the Foreign Intelligence Surveillance Act court, which currently only hears from government lawyers seeking permission to collect on Americans. Intelligence officials have said they might not be opposed to such an advocate.

Feinstein has spoken with Blumenthal about the proposal, but has not publicly committed to supporting his approach. “The idea of introducing another party or having another voice is something [Feinstein’s] looked at and [she] believes there’s a way to accommodate that goal, but there are some limitations on how far that should go,” says one Senate intelligence committee aide.

Feinstein has her own bill that she is preparing with Senator Saxby Chambliss, the ranking Republican on the Senate intelligence committee. Their bill, Feinstein said at a committee hearing last month, would:

Limit access to the Section 215 phone metadata records, expressly prohibit collection of the content of phone calls, and codify the requirement that analysts must have a recent articulable suspicion that a phone number is associated with terrorism in order to even query the database. In addition, we’re looking at reducing the length of time that records can be held and queried. We will also add a requirement that every time NSA determines that there is a reasonable articulable suspicion that a phone number is associated with terrorism, that determination will be sent promptly to the FISA Court so that it can be reviewed. We will also require additional transparency by requiring annual reports on the number of phone numbers determined to have met that reasonable articulable suspicion standard, the number of queries conducted each year, the number of time such queries result in FBI investigative leads each year and the number of probable-cause warrants or FISA Court orders obtained because of intelligence gathered from these queries each year.

Those who oppose the 215 program say this is “window dressing.” But some of those opponents are also clear-eyed about who’s in control on Capitol Hill when it comes to NSA reform. “Everyone’s got a bill [but] most of them have zero chance of moving forward,” says the Senate aide involved in the debate. “At the end of the day, we’ll see primarily Feinstein.”

MORE: The Surveillance Society