So what spying powers has the US lost? The National Security Agency's legal authority to collect the phone records of millions of Americans, part of the US government's counterterrorism efforts, has lapsed, most likely only temporarily.
Why did this happen? The bulk collection of "metadata", which included details of telephone numbers called and the time and duration of calls, was covered by Section 215 of the USA Patriot Act, the sweeping legislation passed after the September 11th, 2001, attacks designed to prevent further terrorist plots.
The provision had a sunset clause expiring just after midnight on Monday. The Senate was unable to agree either a short-term extension of the Patriot Act, prolonging the programme, or pass the new USA Freedom Act that would have put modest restraints on the surveillance programme.
The House of Representatives overwhelmingly passed the Freedom Act, which is supported by President Barack Obama.
Wasn't this the programme that whistleblower Edward Snowden exposed? Yes. The former NSA contractor leaked details of the controversial programme to the media in 2013, leading to a national debate about whether the government was going too far in monitoring the private phone calls of Americans in an effort to track terrorists.
Why was it secret for so long? The NSA started collecting phone "metadata" in secret after 9/11. The Patriot Act was passed in 2002 and signed into law by President George W Bush, giving the powers a legislative footing.
The use of the act to gather telephone information remained secret under applications to the Foreign Intelligence Surveillance Court, where a judge hears requests for access to phone data behind closed doors and makes private rulings that cannot be reported in the media.
USA Today published a report in 2006 disclosing details of the vast trawl of phone records but it was not until the leaks by Snowden, now a fugitive in the eyes of the US government and an exile in Russia, that there was a public debate about the infringement of civil liberties.
So will the Senate support the reformed surveillance programme? Yes, it appears a majority of senators will approve the Freedom Act, despite earlier divisions among Republicans including Senate majority leader Mitch McConnell, who wanted the Patriot Act's bulk collection to be extended for another five years.
Once Republican opponents realised the spying programme was going to lapse, they changed tack and supported the Freedom Act. The Senate voted 77 to 17 on Sunday to consider the new Bill, putting it on a path to be signed into law by Mr Obama, but Rand Paul, the libertarian-leaning Republican senator, blocked a short-term extension of the Patriot Act to retain the bulk collection and other counterterrorism tools in place while the new Bill was being enacted.
Why does Paul want to stop it? The Kentucky senator believes bulk collection is a breach of an American's constitutional rights and the Fourth Amendment that protects private property from government trespass. He has become well known for his long speeches, including one last month that lasted 10 hours, against the NSA surveillance programme. He concedes that the Freedom Act will pass but has claimed a victory for stopping bulk collection, even for a few days.
