What's the Purpose of the N.S.A. Surveillance Program? The White House, desperately trying to defend a huge phone-surveillance program it should have shut down years ago, made an interesting claim on Thursday: Collecting millions of phone records of ordinary citizens allows law enforcement "to discover whether known or suspected terrorists have been in contact" with other possible terrorists, according to Josh Earnest, a deputy press secretary. But that can't be the only purpose of this program, just as it isn't the point of a similarly troubling effort to pore through e-mails and electronic traffic, revealed yesterday by the Guardian and the Washington Post. If counter-terrorism officials are following a suspect and want to know his contacts, they can easily get a court order to obtain the details of his specific conversations. For that matter, they can get permission to wiretap him, too. Investigators might want to keep data longer than a phone or Internet company would, so they can search through years of a suspect's records, but that's too slim a purpose by itself to justify a program of this depth. What's far more likely is that the government is using these records to try to identify terrorists in the first place, not follow them once they are known. Using a high-speed computer, the National Security Agency mines the huge amount of data it gathers daily for patterns that might indicate suspicious activity - calls to a country or a specific person overseas, or unusual communication behavior, like a spike in calls, that coincides with what the N.S.A. is seeing elsewhere. As Senator Saxby Chambliss of Georgia, the ranking Republican on the Intelligence Committee, explained today, the computer tries to flag numbers that might be suspicious, giving investigators justification to demand wiretaps from a federal judge. "All of these numbers are basically ferreted out by a computer," he said, "but if there's a number that matches a terrorist number that has been dialed by a U.S. number or dialed from a terrorist to a U.S. number, then that may be flagged." What the public never learns is how many of those patterns lead to wiretaps of innocent citizens. The Guardian reported that the Internet-search program, known as PRISM, results in 2,000 further reviews of messages every month, which means that government investigators read the actual contents of tens of thousands of messages. That number undoubtedly includes many false hits on people who were not communicating about terrorist plots. But even in the unlikely case that the government never eavesdrops on the wrong people, the cost to civil liberties is still too high. The tiny chance of a useful match cannot justify collecting everyone's phone records, or running searches on millions of e-mail messages and Internet chats. As Jameel Jaffar, deputy legal director of the American Civil Liberties Union, put it today, imagine if the government required every American to report to the government every night who they spoke to, or texted, for how long, and from where. People would be furious, but that's precisely the information the N.S.A. is collecting from telecom companies. And it's precisely why the government desperately wanted to keep the practice a secret. But now the world knows what many members of Congress have kept buttoned up for years. Will Democrats stick to their principles and criticize President Obama for perpetuating a practice that began under President George W. Bush? And will Republicans, happy to find something new to stimulate anger against the White House, demand actual change in a program they defended for years?