A bill that would require authorities to get a warrant to access U.S. emails older than 180 days faces an existential threat as a senator attempts to make it a vehicle for “the FBI’s No. 1 legislative priority,” an expansion of the bureau's authority to take online records without court oversight.

Senate Majority Whip John Cornyn, R-Texas, is a co-sponsor of the Senate version of the email warrant bill, which passed the House unanimously last month, but his proposed changes threaten to kill momentum and perhaps the bill itself.

Cornyn offered two amendments, and other Republican senators more, surprising privacy advocates who hoped to pass the House-approved bill out of the Senate Judiciary Committee on Thursday. Consideration instead was tabled until next month.

Some of the amendments propose warrant carve-outs for emergencies and in cases of consent, concerning civil liberties advocates who want to ensure any such exception is narrowly crafted. But the much more controversial proposal is Cornyn’s amendment to expand FBI national security letter authority.

NSLs are shadowy administrative subpoenas for information issued by the FBI, whose authority to use them was bolstered by the Patriot Act in 2001. The requests often are accompanied by a gag order disallowing the company from which information is sought from discussing it.

Cornyn’s amendment would allow FBI officials, such as agents who lead field offices, to acquire various records without court approval if they determine they are “relevant to an authorized investigation" pertaining to international terrorism or intelligence activities.

Under Cornyn’s amendment, the FBI could point to a person or entity’s name or phone number or account and demand information from a company. It would strike a section of law that allows for taking of “name, address, length of service, and toll billing records” and replace it with permission to get:

Name, physical address, email address, telephone number, instrument number and other similar account identifying information

Account number, login history, length of service (including start date), types of service and means and sources of payment for service (including any card or bank account information)

Local and long distance toll billing records

Internet Protocol (commonly known as ‘IP’) address or other network address, including any temporarily assigned IP or network address, communication addressing, routing, or transmission information, including any network address translation information (but excluding cell tower information) and session times and durations for an electronic communication

The FBI currently can get those records, but aside from “name, address, length of service, and toll billing records” needs court permission under Section 215 of the Patriot Act, opponents of Cornyn's amendment say.

Neema Singh Guliani, legislative counsel at the American Civil Liberties Union, says Cornyn's proposed amendment would allow for the FBI to acquire without court permission “all sorts of records about Internet activity,” including email logs and browser histories.

“The average American would be concerned to know the FBI could issue an NSL and get their browser history [if Cornyn’s amendment becomes law],” she says.

Cornyn’s office did not immediately refute Guliani’s characterization of the records his amendment would affect. An aide says the amendment covers Internet metadata.

If the amendment were adopted, it would almost certainly cause most original backers to bail. Sen. Patrick Leahy, D-Vt., cautioned Thursday that simplicity is key to the bill’s passage.

A similar amendment was included in a spending bill approved Tuesday by the Senate Intelligence Committee, though its precise wording remains secret. It's possible that bill's effect would last just one fiscal year, and it’s unclear if that measure will survive both a full Senate vote and then whatever bicameral deal is struck with the House.

Sen. Ron Wyden, D-Ore., a leading privacy advocate, voted against the intelligence spending bill, saying the measure would take “a hatchet to important protections for Americans’ liberty."

If Cornyn’s amendment is not adopted, however, Senate Republican leadership may be able to block the bill from reaching 60 votes required for consideration, though supports may still come close as GOP senators are among those pushing the reform.

Cornyn said during the short Senate Judiciary Committee gathering Thursday that his controversial amendment would merely “fix an oversight or scrivener's error in the statute that’s needlessly hamstringing our counterintelligence and counterterrorism efforts and is preventing the FISA court from focusing on important work we all want it to do.”

The Texas senator said “this is, according to Director [James] Comey of the FBI, the FBI’s No. 1 legislative priority … and this reflects the Obama administration’s position.”

The characterization of the change as minor is controversial. National security journalist Marcy Wheeler last year described Comey’s characterization of the limitations as “essentially a typo in the law” as an “unmitigated load of bulls---,” pointing to a 2008 opinion from the Justice Department’s Office of Legal Counsel that found the FBI could only demand with an NSL what the law says.

It's unclear what the FBI currently demands using NSLs. In November, the former owner of a small Internet service provider, Nicholas Merrill, won a court battle allowing him to disclose what the FBI demanded without a warrant in 2004. At the time, the FBI demanded a list of information including the user’s address, recent purchases, “all website information registered to the account” and “[a]ny other other information which you consider to be an electronic communication transactional record.”

It's possible the FBI has since then used less creativity interpreting the law to demand electronic communication transactional records as emboldened tech companies resist demands they consider unsupported by law.

The FBI issued more than 400,000 national security letters between 2003 and 2011, between 30,000 and more than 55,000 each year, according to a report issued in 2014 by the Justice Department’s inspector general.

Guliani says it's important to push back against Cornyn’s characterization of the amendment.