There was an odd moment yesterday when President Bush defended, without admitting that it existed, a massive program of collecting data on the domestic telephone records of Americans.

The program, according to a detailed report in USA Today, was administered by the National Security Agency with the cooperation of most of the major American telephone companies.

As described, it is a frightening and unnecessary use of federal power that could lead to major abuses of the privacy rights of innocent Americans.

Bush insists such a program is legal and necessary to fight the war on terrorism. It crosses a dangerous line.

It's difficult to prove how collecting the phone records of "tens of millions" of Americans directly aids in the fight against terrorism.

This program, unlike the unauthorized intercepts of calls between Americans and suspected terrorists overseas, does not actually tap into the calls themselves.

Rather, it is a "data mining" exercise designed to detect suspicious patterns. Computers could determine who is calling individuals under suspicion and then could pump out other calling patterns.

Most American citizens might accept, if reluctantly, such a program if it is kept squarely within the four corners of investigations into potential terrorist activity within the United States.

But there is no way to know what information is collected and how it will be used.

Members of Congress are rightly calling for a full-scale investigation. It is imperative that the Bush administration explain in detail precisely what they are looking at, how the information is being used and what safeguards against abuse are in place.

If those answers are not forthcoming or sufficient, then Congress should insist that the program be scrapped.

There are no guarantees in dealing with such a large-scale collection of data. The material could be used for domestic surveillance of political enemies, whistleblowers or social activists.

Unlikely? Think again. Many (think Richard Nixon and FBI Director J. Edgar Hoover) kept robust enemies lists and would have drooled over a sophisticated computerized tool such as this.

If authorities have a legitimate belief that the calling patterns of an individual are important to their investigation, it is easy enough to get a judge to grant that authority.

But a sweeping broad-brush program such as the one described by these reports goes too far and poses serious dangers.