Senate supporters of legislation requiring authorities to get a warrant to access U.S. emails older than 180 days pulled the plug Thursday after a committee appeared likely to repurpose the bill to expand the FBI's authority to take records without court approval.

The House unanimously approved its version of the email warrant bill last month, but the reform now is on ice in the upper chamber.

Sen. Mike Lee, a Utah Republican and the bill's primary sponsor in the chamber, withdrew it from consideration in the face of an amendment from Senate Majority Whip John Cornyn, R-Texas, that would expand the records the FBI can acquire using a national security letter, or NSL.

NSLs are shadowy administrative subpoenas issued by the FBI to demand records it claims are "relevant to an authorized investigation" into terrorism or intelligence activities. The requests often include a gag order banning the recipient from discussing them.

The leading Democrat behind the bill, Sen. Patrick Leahy of Vermont, agreed with yanking it from Senate Judiciary Committee consideration, saying at a Thursday committee meeting the amendment was a "poison pill" he doubted would pass the House.

The email warrant bill was expected to win swift committee passage before Cornyn offered the amendment – which he's described as "the FBI's No. 1 legislative priority" – forcing the bill to be tabled until after a recess.

Lee spokesman Conn Carroll tells U.S. News that a vote-counting assessment of support for Cornyn's amendment prompted the decision to withdraw the bill.

Cornyn on Thursday argued his amendment would cover "just IP addresses and email addresses – it's metadata." Privacy advocates say it also would encompass email logs and browser history.

"It strikes me as odd that somebody could call an amendment a poison pill when I believe the amendment enjoys the support of a majority of this committee," Cornyn said. "I would remind everyone this is [FBI] Director [James] Comey's No. 1 legislative priority."

Lee said if the amendment was so popular, Cornyn should find another legislative route or run it as a standalone bill.

Cornyn last month said the amendment would merely "fix an oversight or scrivener's error in the statute that's needlessly hamstringing our counterintelligence and counterterrorism efforts." But that characterization is controversial.

National security journalist Marcy Wheeler last year wrote it was an "unmitigated load of bulls---" for Comey to claim NSL limitations were "essentially a typo in the law," pointing to a 2008 opinion from the Justice Department's Office of Legal Counsel limiting what the FBI can demand without court approval to what the law says.

An amendment similar to the one that nixed the email warrant bill was included in a bill approved last month by the Senate intelligence committee. It's possible that bill's effect would last just one fiscal year, and it's unclear if the measure will survive both a full Senate vote and then whatever bicameral deal is struck with the House.

Carroll says it's "up to Cornyn" on when the email privacy legislation will be considered. Thursday's action "means the bill isn't going anywhere until Cornyn drops his amendment," he says.