Republican leaders in Congress on Sunday said a potential U.S.-led accord with Iran will let the Islamic Republic continue working on a nuclear weapon, and predicted a tough fight to win legislative approval.

House Speaker John Boehner said President Barack Obama has backed away from guidelines set for the talks, and Senate Majority Leader Mitch McConnell said “a very hard sell” will be needed for a deal worked out by the U.S. and five other countries to pass Congress. Leaders of the Senate Foreign Relations Committee also voiced concern.

“We already know that it’s going to leave Iran as a threshold nuclear state,” McConnell of Kentucky said on the “Fox News Sunday” broadcast. “No deal is better than a bad deal,” Boehner of Ohio said on CBS’s “Face the Nation.”

Diplomats continued to work late into the day Sunday in Vienna seeking to bridge gaps over a United Nations embargo on arms sales to Iran and other issues that have made an agreement elusive. U.S. Secretary of State John Kerry said he and his Iranian counterpart Mohammad Javad Zarif were “getting to some real decisions” and voiced optimism that an accord was within reach.

Should a deal be reached by Monday, the latest in a series of deadlines set by negotiators, legislation passed in May gives Congress 60 days to review and hold hearings. During the time, Obama can’t waive or reduce sanctions against Iran.

Lawmakers then could vote on a joint resolution to approve or reject the deal, though they also may not act at all.

Obama Veto

A vote to disapprove the pact wouldn’t necessarily be the end. Obama could veto the resolution, and the House and Senate would each need a two-thirds majority to override his veto.

“He knows that the resolution of disapproval is likely to be introduced, is very likely to pass and very likely to get over 60 votes,” McConnell said. “He’ll have to get at least 34 senators to go forward.”

Senate Foreign Relations Committee Chairman Bob Corker and the committee’s top Democrat, Robert Menendez of New Jersey, said they are uncomfortable with the course of negotiations.

“I am concerned about where we are going,” Corker said on NBC’s “Meet the Press.” broadcast. “We have moved toward managing their proliferation” instead of preventing Iran from getting a nuclear weapon.

On ABC’s “This Week,” Menendez said he is “anxious” about the shift.

‘Rolling Back’

“We have gone from preventing Iran having a nuclear ability, to managing it,” Menendez said. “What we are doing is basically rolling back sanctions for -- not rolling back Iran’s elicit nuclear infrastructure, but rolling back sanctions for verification.”

One of the administration’s harshest critics on Iran, Senator Tom Cotton, said the U.S. should have walked away from the negotiating table long ago.

“Iran is part of the problem with Islamic State,” Cotton, an Arkansas Republican, said on “Face the Nation.” “The president’s commitment to a nuclear deal with Iran has tied our hands in Iraq.”

An accord would lift economic sanctions that have crippled Iran, holder of the world’s fourth-largest oil reserves and second-biggest natural gas stockpile. As the high-stakes negotiations grind on, all sides say they’ve never been closer to reaching agreement. A Western official involved in the talks said Sunday a draft accord hasn’t been completed yet.