Freedom of the press isn’t valuable for the good things it accomplishes as much as for the bad things it prevents. That observation is as true today as when Alexis de Tocqueville first made it in the 19th Century.

It’s also at the heart of the argument for legislation that both houses of Congress are considering with backing by lawmakers from both parties. The so-called federal shield law would protect news reporters from having to disclose certain information they acquire in the course of their jobs. Conspicuously, they would not have to reveal the identities of sources who provide information confidentially.

“Without such protection,” said Sen. Richard Lugar, R-Ind., prime sponsor of one proposed shield law, “many whistle-blowers will refuse to step forward and reporters will be disinclined to provide our constituents with the information that they need to know.”

A string of embarrassing incidents of journalistic misconduct in recent years has done nothing to make Lugar’s and his colleagues’ job easier. Credibility in the news has suffered. Yet the founding fathers didn’t craft the First Amendment because they had inviolate trust in the newspapers of their own day (Jefferson called them “licentious” and “polluted”), let alone ours.

Perfect or not, an independent and aggressive press serves an essential purpose in a self-governing society. Citizens can hold their government accountable only if they know what it’s up to, yet public officials at all levels of government command powerful resources that help them conceal and control the flow of information citizens receive.

Journalists serve a critical role, peeling back the shrouds of secrecy that sometimes veil public misconduct. Their invaluable ally in that process is the principled insider, the person who knows what’s going on and weighs the ethical considerations of honesty against the personal risk to career, liberty or even life, and opts to provide vital information.

The ability of a reporter to grant anonymity to sources who provide credible information is often the only way to get important information to the public that has a right and a need to know it. Without a shield law, however, reporters must be willing to put their own freedom at risk to carry out that promise.

At present, more than a half-dozen pending cases involve actual or threatened jailing of U.S. journalists who are protecting sources. The most conspicuous is that of New York Times reporter Judith Miller and Time Magazine reporter Matthew Cooper, fighting a contempt order over their refusal to reveal who talked to them about the identity of undercover CIA agent Valerie Plame. New England television reporter Jim Turiconi was placed under house arrest because he wouldn’t tell who gave him an FBI surveillance tape in a bribery case.

In an era when terrorism concerns are being emphasized at the expense of Americans’ civil liberties, the need for a government watchdog was never greater.

Thirty-one states and the District of Columbia have their own shield laws (Washington and Idaho are not included), but only a federal law will assure American freedom of the press the security it needs to continue performing the service Tocqueville recognized.