Privacy-minded senators on Wednesday blocked an amendment that would give the FBI power to take internet records, including browsing histories and email metadata, without a court order. But the victory may be fleeting.

Just one vote kept the measure from clearing a 60-vote procedural hurdle, and political arm-twisting may soon result in a second vote. Senate Majority Leader Mitch McConnell, R-Ky., switched his vote to "no" to allow reconsideration in the near future. That made the final tally 58-38, with four senators not voting.

Critics of the proposed expansion of the FBI's ability to demand records with national security letters, or NSLs, are urging opponents to flood their senators with calls. There were some unexpected "yes" votes, such as Sen. Ted Cruz, R-Texas, who they hope to flip as some of the four senators who did not vote are viewed as tougher sells.

"It's obviously a good thing that this didn't move forward in the Senate," says Neema Singh Guliani, legislative counsel at the American Civil Liberties Union. "This would be an expansion of the Patriot Act and a very substantial one that would allow the FBI to get what many people consider their most sensitive information."

"It's important that the public contact their senators and say, 'We don't want this expansion of the Patriot Act,'" she says. "There were a lot of members who voted in favor who you wouldn't expect. This is a situation where you could see a lot of pressure on members to change their votes, which is why it's important the public understands the stakes here."

The amendment to a multiagency spending bill would allow the FBI to use NSLs to force companies to turn over “electronic communications transactional records," sometimes referred to as ECTRs, when it claims they are relevant to an investigation into terrorism or espionage. NSLs are administrative subpoenas that don’t require court approval and often come with a gag order.

Critics say the FBI already can get ECTR records if it convinces a judge there's good cause or if there's an emergency and it seeks retroactive court review.

“When most people hear ECTR, they go, ‘What’s an ECTR?’ And of course they do," says Robyn Greene, policy counsel at the Open Technology Institute. "ECTRs are not records that people are familiar with. When you send an email or go to a website, you think about the content you are sending or receiving, not that there's a trail you are leaving that if the government accessed would reveal your entire digital fingerprint."

Greene, who opposes the amendment, says "they were beat, but they may try again."

The surveillance-enhancing amendment is part of a third attempt to get the NSL expansion through the Senate. A first attempt successfully attached it to the annual intelligence authorization bill, which the Senate intelligence committee approved behind closed doors with a lone "no" vote from Sen. Ron Wyden, D-Ore. The underlying bill hasn't been considered by the full Senate. A second attempt killed a bill that would have required warrants for U.S. emails when there were enough votes to attach the measure in the Senate Judiciary Committee.

On Tuesday, a prominent supporter of the legislation, Sen. John Cornyn, R-Texas, said the authority could have helped the FBI apprehend Orlando mass murderer Omar Mateen, who the agency had twice investigated years before he killed 49 people at a gay nightclub on June 12 in the worst mass shooting in modern U.S. history. Cornyn said it might have showed Mateen’s email contacts and that he was watching sermons posted online of radical cleric Anwar al-Awlaki.

Wyden argued on the Senate floor Wednesday that the amendment violates the Fourth Amendment’s protections and that it’s unnecessary because the surveillance-reforming USA Freedom Act enacted last year “allows the FBI to demand all of these records in an emergency and then go get court approval after the fact. So unless you’re opposed to court oversight, even after the fact, there’s no need to support this amendment.”

One of the amendment’s sponsors, Sen. Richard Burr, R-N.C., said there’s no evidence the authority would have prevented the Orlando shooting. He said “this is simply to provide law enforcement with tools to fulfill their mission, which is to keep America safe" and would shorten what can be a monthlong process of the FBI requesting permission from the Foreign Intelligence Surveillance Court to a “one-day process” with an NSL – something Wyden said was untrue in emergencies, given the Freedom Act provision.

The Justice Department's Office of Legal Counsel concluded in 2008 current law does not authorize taking of ECTR records with an NSL, which the FBI had been requesting anyhow. Burr said most companies, however, provided the records to the FBI regardless until 2010, “when a general counsel in one company decided to buck the system.”

Despite the Justice Department opinion, the FBI was still demanding ECTR records as late as 2013, as indicated in an NSL published by Yahoo! this month, after implementation of Freedom Act reforms requiring the FBI to review open-ended gag orders and determine if they are no longer needed.

Nicholas Merrill, the owner of now-defunct Calyx Internet Access who in November became one of the first people given permission by a court to speak freely about receiving a NSL for customer information, which he fought beginning in 2004, says the expansion should be resisted.

“The fact that the government is now attempting to legalize the demand for electronic communications transactional records that they demanded of me back in 2004 is a tacit admission that what they did with all of the roughly 500,000 NSLs issued since then was illegal and overreaching,” Merrill says.

Merrill says the FBI has attempted to avoid court review whenever possible by dropping NSL requests, such as happened in his case.

“The reason the government getting access to electronic communications transactional records is bad is because they paint a vivid picture of First Amendment-protected speech and association online, without requiring any particularized suspicion of wrongdoing,” he says. “In other words, national security letters are used to go on fishing expeditions.”

Whether the measure can pass the House of Representatives is unclear. Since whistleblower Edward Snowden’s 2013 disclosures about mass surveillance, the House has been more deferential to privacy pushes – but following the Orlando shooting, momentum may have stalled, with House members rejecting an effort to ban “backdoor” NSA surveillance after passing the amendment in 2015 and 2014.

“It's a question of constitutional values, and of checks and balances on executive authority,” says Steven Aftergood, a government secrecy expert at the Federation of American Scientists.

“There are all kinds of intrusive law enforcement techniques, including warrantless search and seizure, that might be useful in reducing crime,” he says. “But the constitutional path is to require checks and balances on their use. If more authority is needed, fine – but only with a corresponding increase in external oversight and accountability. The NSL legislation does not provide for that.”