For Barack Obama - a President who prided himself on his liberal credentials - this leak is a potentially devastating revelation since it exposes him to attack on two fronts - both the libertarian right and the liberal left.
It is not clear how wide the NSA data-mining project goes, it's effectiveness as a counter-terrorism tool in identifying potential terrorist or criminal cells or whether it has been used for any other purposes. It appears from previous reports that the NSA's data-mining operation has long been suspected - but this is the first clear-cut proof, in the shape of a highly unusual leak from the secretive Foreign Intelligence Service Court (Fisa), that the practice is occurring. It also shows the Obama Administration has continued a practice begun under the Bush Administration.
A report in USA Today from 2006, quoting anonymous intelligence officials, alleged that the NSA been "secretly collecting the phone call records of tens of millions of Americans" and that the agency was using the data to "analyse calling patterns in an effort to detect terrorist activity".
Cindy Cohn, a lawyer at the Electronic Frontier Foundation, a civil liberties organisation that has accused the Government of operating a secret dragnet surveillance programme, told the Washington Post "I don't think Congress thought it was authorising dragnet surveillance" when it passed the 2001 Patriot Act.
Cohn said: "I don't think Americans think that's okay. I would be shocked if the majority of Congressmen thought it's okay."
Former Vice-President Al Gore tweeted: "In digital era, privacy must be a priority. Is it just me, or is secret blanket surveillance obscenely outrageous?"
Over the next few days and weeks, expect a fierce and polarising debate over just what Americans do feel is acceptable, in the name of their national security.