Despite those and other obstacles, negotiators appear increasingly likely to clinch an historic deal to restrict Iran’s nuclear program for at least a decade in exchange for relief from sanctions, Western and Iranian officials said.

U.S. officials, including Obama, have long said they see at best a 50-50 chance of getting a deal with Iran.

That remains the official line, but diplomats close to the talks tell Reuters the chances are higher than that as foreign ministers and other negotiators head to Vienna next week for the final stage of a nearly two-year process.

Driving the cautious optimism, they say, is not so much progress made in overcoming sticking points as the intense political pressure on the U.S. and Iranian delegations to reach a deal that would end the 12-year nuclear stand-off between Iran and the West.

“We can’t rule out failure, but ... it seems more likely that we will get something. Not by June 30, but perhaps in the days that follow,” a senior Western official told Reuters, referring to the planned deadline for a deal.

Another Western official said: “I think there will be an agreement because the two most important players need it.”

A preliminary agreement reached in early April left major differences for negotiators to bridge, including the verification regime to ensure Iranian compliance with a deal and the timetable for lifting sanctions.

Securing a nuclear deal is politically vital for Obama.

Obama must win approval for the deal from the Republican-controlled Congress, where many fear the deal will boost Iran’s as a regional power and increase its threat to U.S. allies such as Israel and Saudi Arabia. Officials close to the talks say Obama expects Congress to oppose the deal and is prepared to use his veto to ensure it is not derailed.