Republican leaders roundly condemned a potential Iran nuclear deal under negotiation in Vienna on Sunday, saying that Iran will continue to build a nuclear weapon despite any accord reached.

Any deal reached in Vienna that does not provide verification Tehran has restricted its nuclear program in exchange for sanctions relief is useless, GOP leaders said Sunday.

"We already know that it's going to leave Iran as a threshold nuclear state," said Senate Majority Leader Mitch McConnell, R-Ky., on "Fox News Sunday." He said "a very hard sell" will be needed to convince Congress to approve such a deal.

President Obama has walked back every part of the deal he originally set out to achieve, and the U.S. completely abandoned the guidelines set out for the talks, said House Speaker John Boehner, R-Ohio, Sunday.

"No deal is better than a bad deal," he said on CBS's "Face the Nation," adding that a "nuclear standoff" would be better than "legitimizing this rogue regime."

Senate Foreign Relations Committee Chairman Sen. Bob Corker said an Iran deal concerns him because the Islamic Republic will "cheat by inches." Iran desires nuclear weapons regardless of what diplomatic deal Secretary of State John Kerry is able to attain, said Corker on Sunday.

"We know they were building a bomb, we just don't know how far along they were," Corker said on NBC's "Meet the Press."

"I am concerned about where we are going," he said, adding that the U.S. has moved from preventing Iran getting a nuclear weapon to simply "managing their proliferation."

A senior Democrat on the Senate Foreign Relations Committee shared Corker's concerns and thought negotiations had "started off with the wrong premise."

"We have gone from preventing Iran having a nuclear ability, to managing it," said Sen. Robert Menendez, D-N.J. "What we are doing is basically rolling back sanctions for — not rolling back Iran's illicit nuclear infrastructure, but rolling back sanctions for verification."

Diplomats in Vienna signaled Sunday that they may have reached a provisional agreement and would make a formal announcement Monday, after the deal was reviewed by the seven nations at the talks — Iran, the United States, Russia, China, Germany, France and Britain.

The deal would then be sent to Congress, which has 60 days to review and approve it. Obama cannot reduce sanctions on Iran during that time.

If Congress does not approve the deal, they could offer a resolution of disapproval. The House and Senate would have to muster a two-thirds majority to override the president's 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 Sunday. "He'll have to get at least 34 senators to go forward."