By Richard Cowan and Douwe Miedema
WASHINGTON (Reuters) - The top Republican in the U.S. Senate cast doubt on Sunday on whether President Barack Obama would be able to win approval in Congress for any nuclear deal with Iran, and some Democrats also expressed reservations.
"I think it's going to be a very hard sell, if it's completed, in Congress," Senate Majority Leader Mitch McConnell told "Fox News Sunday". "We already know it's going to leave Iran as a threshold nuclear state."
The U.S. Congress could play a key role in the future of any deal to curb Iran's nuclear program, with negotiations reaching a critical juncture in Vienna on Sunday.
Obama can enter a deal with Iran, which Congress would review. If the review went against him, Obama could veto the disapproval legislation. Congress could then try to reverse his veto, which is difficult. But if it succeeded, its disapproval would take away Obama's ability to temporarily waive many U.S. sanctions on Iran.
Robert Menendez of New Jersey, a leading foreign policy voice among Senate Democrats, said the prospect of a deal made him "anxious," saying the talks had moved from preventing Iran from having a nuclear capability, to managing it.
Western diplomats say the goal of the deal is to increase the time it would take for Iran to produce enough enriched uranium fuel for a weapon to at least one year, from current estimates of 2-3 months. In return for curbs on its nuclear activities, Iran would get relief from economic sanctions.
Menendez, interviewed on ABC's "This Week" program, did not rule out supporting a deal.
Many congressional Republicans, including McConnell, have criticized the negotiations, saying the United States should be increasing economic sanctions against Iran. Tehran maintains its nuclear work is solely for peaceful purposes.
Referring to the congressional review process, McConnell said: "He (Obama) will have to get at least 34 votes" in the 100-member Senate to sustain his veto, adding that he hoped Democrats would resist a "strong pull" not to buck Obama.
The debate in Washington over any deal would happen with the 2016 U.S. presidential campaign already under way.
One Republican candidate, Senator Lindsay Graham, said on CNN's "State of the Union" the negotiations should be left to the next president, who would take office in January 2017.
"I think a good outcome is to basically leave the interim deal in place" with Iran until then, Graham said.
House Speaker John Boehner said failure of the talks would not be a bad outcome.
On CBS's "Face the Nation," Boehner said: "If, in fact, there's no agreement, the sanctions are going to go back in place," prodding Iran to "abandon their efforts to get a nuclear weapon, and stop being the largest state sponsor of terrorism in the world."
(Editing by Ruth Pitchford and Robin Pomeroy)