The legal authority for US spy agencies' collection of Americans' phone records and other data has expired after the US Senate failed to pass legislation extending the powers.

The Senate voted on Sunday to move ahead with reform legislation that would replace the bulk phone records programmes revealed two years ago by Edward Snowden, the former National Security Agency (NSA) contractor.

The vote came after an exhaustive debate pitting Americans' distrust of intrusive government against fears of violent attacks.

Opinion: Patriot Act ends but the spying goes on

It was a victory for Democratic President Barack Obama, who had pushed hard for Congress to advance the reform measure.

He has called it a compromise that addressed privacy concerns while preserving a programme his administration describes as important to protect the country from attack.

But final Senate passage was delayed until at least Tuesday morning by objections from Senator Rand Paul, a libertarian Republican presidential hopeful who has fulminated against the NSA programme as illegal and unconstitutional.

Resumption likely

As a result, the government's collection and search of phone records was set to terminate at midnight when provisions of a post-9/11 law known as the USA Patriot Act expire.

Still, eventual resumption of the phone records programme in another form appeared likely after the Senate voted 77-17 to take up the reform legislation, called the USA Freedom Act.

In a recent poll, 60 percent of Americans - Republican and Democrats - said they believed the Patriot Act should not be reauthorised in its current form.

Approximately 80 percent said they found it concerning that their government was collecting and storing their personal information.

"Over the last two years we've seen the depth of surveillance abuses occurring and given the extent of these abuses, we need more substantial reform," said Neema Singh Guliani, the legislative counsel of the American Civil Liberties Union.

"The Freedom Act takes modest steps forward but but we believe it needs to be improved.

"It needs [to] have more enhanced transparency and more procedures that ensure information collected is purged by the government."

Some independent experts believed that letting the provisions expire was a step in the right direction.

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"The sunset is definitely a good thing for surveillance reform, but it's a very small victory," said Joshua Kopstein, a cyberculture journalist and researcher based in New York City.

"There's been a lot of talk about 'the Patriot Act expiring' but the truth is only a very, very tiny percentage of the law is actually going away," he told Al Jazeera.

"A lot of these authorities are redundant, so there are still many ways for the NSA to do mass surveillance without a warrant."

Kopstein, who specialises in internet law and order, surveillance and government secrecy, said one example is Section 214, which the government can use to collect email metadata in bulk.

"They can also use National Security Letters to force your service provider to give them your data and then gag the provider from telling you - all without a warrant," he said.