Tracking whom Americans are calling, for how long they speak, and from where, can reveal deeply personal information about an individual. Using such data, the government can discover intimate details about a person’s lifestyle and beliefs — political leanings and associations, medical issues, sexual orientation, habits of religious worship, and even marital infidelities. Daniel Solove, a professor at George Washington University Law School and a privacy expert, likens this program to a Seurat painting. A single dot may seem like no big deal, but many together create a nuanced portrait.

The effect is to undermine constitutional principles of personal privacy and freedom from constant government monitoring. The American Civil Liberties Union filed a lawsuit on Tuesday, challenging the program’s constitutionality, and it was right to do so.

The government’s capacity to build extensive, secret digital dossiers on such a mass scale is totally at odds with the vision and intention of the nation’s framers who crafted the Fourth Amendment precisely to outlaw indiscriminate searches that cast a wide net to see what can be caught. It also attacks First Amendment values of free speech and association.

In a democracy, people are entitled to know what techniques are being used by the government to spy on them, how the records are being held and for how long, who will have access to them, and the safeguards in place to prevent abuse. Only then can they evaluate official claims that the correct balance between fighting terrorism and preserving individual liberty has been struck, and decide if they are willing to accept diminished privacy and liberty. If Americans have been slow to recognize the dangerous overreach of the N.S.A.’s phone surveillance, it is largely because they have scant information to judge the government’s conduct.

Even if most Americans trust President Obama not to abuse their personal data, no one knows who will occupy the White House or lead intelligence operations in the future. The government’s capacity to assemble, keep and share information on its citizens has grown exponentially since the days when J. Edgar Hoover, as director of the F.B.I., collected files on political leaders and activists to enhance his own power and chill dissent. Protections against different abuses in this digital age of genuine terrorist threats need to catch up.