Acrimony erupted in the US Senate over a major surveillance overhaul on Tuesday, hours before legislators are due to vote on moving it forward, as opponents labeled it a gift to terrorism.

The incoming Senate majority leader, Mitch McConnell of Kentucky, sharply warned that the USA Freedom Act, which, if passed, would be the first law to constrain the National Security Agency in decades, would cripple US intelligence against the Islamic State (Isis) in Iraq and Syria.

“This is the worst possible time to be tying our hands behind our back,” said McConnell, who will become majority leader in January.

“At the moment, we should not be doing anything to make the situation worse.”

The USA Freedom Act passed the House of Representatives in May and enjoys what the White House on Monday called its “strong support”. The first bill to reform government surveillance since Edward Snowden’s disclosures 18 months ago would divest the NSA of its ongoing daily collection of practically all Americans’ phone data.

Last week, the current Senate majority leader, Democrat Harry Reid of Nevada, agreed to move forward with a vote on the bill. Should the Senate not pass it this week, the bill will likely die, leaving successor reform bills with an uncertain future in the next Congress – although all sides agree that reauthorizing a critical surveillance provision of the Patriot Act next year will be similarly uncertain.

Yet the new attack on the bill as a boon to terrorism could upend supporters’ already difficult legislative path to secure 60 votes for it. Few in the Senate are confidently predicting the outcome of the vote, scheduled for Tuesday evening, and privacy groups were encouraging their members to bombard their senators with phone calls.

While tech companies and most civil libertarian groups back the Senate version of the USA Freedom Act, few privacy advocates believe it would decisively move the US out of the era of dragnet surveillance. Only the domestic phone records database would be jettisoned, and even then the NSA would be able to acquire from telcos thousands of call records based on a single court order.

The NSA and the Federal Bureau of Investigation will still retain the power to warrantlessly search through the NSA’s vast collections of international communications information for Americans’ identifying information. No foreign nationals would receive greater privacy protections from the USA Freedom Act, nor would the NSA be prevented from undermining digital encryption.

While the Obama administration and the intelligence agencies have backed the bill, their support came only after officials and legislators met in August to loosen its provisions restricting what data the NSA will be permitted to collect. That effort, which followed passage in a critical House committee, cost the bill the support of tech firms and privacy groups, although most have rallied to the Senate version.

But the rise of Isis in Iraq and Syria has given new ammunition to the USA Freedom Act’s many critics in the Senate. Although the NSA has given up on the baseless argument that the US phone records dragnet has prevented domestic terrorist attacks, senators and ex-NSA officials are insisting doing away with it would be a gift to terrorism.

“We live in a dangerous world, and the threat by Isil only makes it more so,” McConnell said, using an alternative acronym for Isis. A similar argument, advanced by former NSA director and post-9/11 dragnet surveillance architect Michael Hayden, received prominent billing in the Wall Street Journal ahead of the vote.

Vermont Democrat Patrick Leahy, the major Senate advocate for the bill, rejoindered: “The NSA has had every single tool available while Isil built up its strength and not one single alarm bill rang, so let’s deal with the facts, not hypotheticals.”

Reid said on Tuesday that he would not object to an expected avalanche of amendments to the bill, coming from both NSA supporters and privacy advocates, all of which render its fate uncertain. Presidential hopeful Rand Paul, a Kentucky Republican, is considering an amendment strengthening the bill’s privacy and transparency measures.

One amendment that Reid supports is a holdover from the Stop Online Privacy Act (Sopa) that would criminalize the streaming of copyright-protected content, a provision strongly opposed by digital privacy advocates. Reid backs the measure, first reported by Techdirt, as a boon to his supporters in the Ultimate Fighting Championship MMA league. But he is said not to be prioritizing it ahead of other amendments more directly relevant to the USA Freedom Act, and is unlikely to offer it unless he first secures the votes for passage, another tall order.