Once again, a thorough and independent analysis of the government’s dragnet surveillance of Americans’ phone records has found the bulk data collection to be illegal and probably unconstitutional. Just as troubling, the program was found to be virtually useless at stopping terrorism, raising the obvious question: Why does President Obama insist on continuing a costly, legally dubious program when his own appointees repeatedly find that it doesn’t work?

In a 238-page report issued Thursday afternoon, the Privacy and Civil Liberties Oversight Board, a five-member independent agency, called on the White House to end the phone-data collection program, for both constitutional and practical reasons. The board’s report follows a Dec. 16 ruling by Federal District Judge Richard Leon that the program was “almost certainly” unconstitutional and that the government had not identified “a single instance” in which it “actually stopped an imminent attack.”

Two days later, a panel of legal and intelligence experts convened by Mr. Obama after the disclosures by Edward Snowden echoed those conclusions in its own comprehensive report, which said the data sweep “was not essential to preventing attacks” and called for its end.

The growing agreement among those who have studied the program closely makes it imperative that the administration, along with the program’s defenders in Congress, explain why such intrusive mass surveillance is necessary at all. If Mr. Obama knows something that contradicts what he has now been told by two panels, a federal judge and multiple members of Congress, he should tell the American people now. Otherwise, he is in essence asking for their blind faith, which is precisely what he warned against during his speech last week on the future of government surveillance.