The Senate’s top Republican is warning lawmakers against overhauling the nation’s surveillance programs, darkening the prospects for a reform bill that hits the chamber floor on Tuesday.

Senate Minority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellOcasio-Cortez to voters: Tell McConnell 'he is playing with fire' with Ginsburg's seat McConnell locks down key GOP votes in Supreme Court fight Video shows NYC subway station renamed after Ruth Bader Ginsburg MORE (R-Ky.) is “strongly opposed” to the USA Freedom Act, he said on the Senate floor on Tuesday, because it would “end one of our nation’s critical capabilities to gather significant intelligence on terrorist threats.”

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The bill is coming up for a procedural vote on Tuesday evening, and many watchers are unsure it has the 60 votes needed to move forward. McConnell’s fierce opposition only hinders its chances to win more Republican backers. Many GOP lawmakers have already said they oppose the bill.

“This is the worst possible time to be tying our hands behind our backs,” the minority leader said.

The threat from the Islamic State in Iraq and Syria (ISIS) “is real,” he added, and “different from what we’ve faced before.”

“At a moment when the United States is conducting a military campaign to disrupt, dismantle and defeat ISIL, now is not the time to be considering legislation that takes away the exact tools we need to combat ISIL,” he said, using an alternate acronym for the terror group.

The bill, which was written by Sen. Patrick Leahy (D-Vt.), would end the National Security Agency’s ability to collect bulk records about Americans’ phone calls, add a team of civil liberties advocates to the secretive federal surveillance court and allow companies to make additional disclosures about the information they hand over to the government. The spy agency’s phone records program was the most controversial element of Edward Snowden’s leaks last summer.

The NSA program, McConnell said, is already guarded by “detailed oversight procedures” to protect people’s privacy.

He accused Democrats of trying to “rush” the bill forward by skipping a committee vote and bringing the bill straight to the Senate floor.

While many Republicans have opposed the bill, it has won crucial support from a handful of lawmakers on the right, including Sens. Ted Cruz (R-Texas) and Mike Lee (R-Utah).

Supporters of the bill quickly denied that it would hamstring U.S. intelligence agencies. Obama administration officials including Director of National Intelligence James Clapper, they noted, have supported the measure.

Cruz would not respond directly to McConnell’s remarks, but told reporters on Tuesday that he hoped to see “bipartisan support” for the bill, which would protect people’s privacy “while at the same time maintain the ability to protect our national security and keep our country safe.”

“It is important that we protect the constitutional rights of American citizens and that we stop this administration’s bulk collection of personal data from law abiding citizens,” he added.

— Updated at 12:37 p.m.