At a minimum, the news that the National Security Agency is engaged in an extremely broad effort to gather the phone records of millions of Americans would seem to warrant a full public debate about the program. The problem is that we can’t really have any such debate, because so much of the program is shrouded in secrecy. So step one is to secure more transparency around it — and the legal rationale the Obama administration is claiming for it.

That’s the key takeaway that was offered to me by Senator Tom Udall of New Mexico, who is in a good position to address this story, because he was one of the Senators who first raised the possibility back in 2012 that this could happen.

“The news that the government issued a blanket order for Americans’ phone records is deeply disturbing,” Udall told me. “We need to know what they are doing, and what their legal basis for doing it is.”

Last year, Udall asked the Director of National Intelligence to provide “as much information as possible” to Congress and the public about the 2012 reauthorization of the FISA Amendments Act of 2008, which gave the government sweeping surveillance authority like that in the Patriot Act. Udall worried that it could lead to surveillance overreach such as what we may be seeing now.

Meanwhile, another Senator Udall — Mark Udall of Colorado — along with Senator Ron Wyden, last year asked the Obama administration for more information about its secret interpretation of Section 215 of the Patriot Act, which authorizes the government to demand information from telephone carriers about calls. “There is now a significant gap between what most Americans think the law allows and what the government secretly claims the law allows,” the Senators wrote in a letter to Attorney General Eric Holder, one that was not answered to their satisfaction.

The Guardian’s Glenn Greenwald reported late yesterday that under an order by the secret Foreign Intelligence Surveillance Court, the government is collecting the phone records of millions of Verizon customers in the United States — raising questions about the proper limits on the government’s information gathering power.

Today the Obama administration, and Senators in both parties, defended the surveillance program, arguing that it only collects metadata, doesn’t allow for listening in on calls, and that it is a critical tool in allowing the intel community to monitor the activities of suspected terrorists. But Udall noted that the sheer lack of information about the program’s scope and functioning made any real public evaluation of it — or debate about it — impossible. Indeed, he suggested more transparency could precede any decision about the program itself — striking, in effect, a balance between officials’ defense of the program and the public’s right to know about it.

“The important thing is to make all of this public,” Senator Tom Udall told me. “Let the American people know what you are doing. The American people have a right to know this. We can have the debate of national security versus privacy.”

Udall said he would renew his push for a legislative solution that would bring about more transparency. He said the Director of National Intelligence should be required to report to Congress a full assessment of the extent of communications monitored and the degree to which info on wholly domestic communications has been collected — in short, an explanation of just how sweeping the program really is. If the President thinks divulging anything in the assessment is a threat to national security, he can redact it, Udall said.

Some have argued that what Obama is doing is not akin to what happened under President Bush, noting that Bush was criticized for warrantless wiretapping with no approval from FISA, while the current order was authorized by FISA but didn’t involve wiretapping. But Udall said there are still plenty of troubling similarities. “This is a very broad collection of Americans’ private information,” Udall said.

What does seem immediately clear is that we need to know more. “I’m asking the administration to be forthright about what they are doing, and they are not being forthright,” Udall said. “We still aren’t getting answers about how broad and sweeping this is. The first thing you need for this debate is the answer to that question.”

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UPDATE: Post edited to better explain the contributions to the debate made by both Udalls — Tom and Mark.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.