Iran and six world powers are reportedly on the brink of an agreement on curbing the country's nuclear program. and AP reports:

U.S. Secretary of State John Kerry, who on Thursday had threatened to walk away from the negotiations, said Sunday that "a few tough things" remain in the way but added "we're getting to some real decisions."

En route to Mass at Vienna's gothic St. Stephens Cathedral, Kerry said twice he was "hopeful" after a "very good meeting" Saturday with Iranian Foreign Minister Mohammad Javad Zarif, who had Muslim services Friday. The two met again early Saturday evening.

French Foreign Minister Laurent Fabius also was cautiously optimistic, telling reporters Sunday:  "I hope that we are finally entering the last phase of this negotiation."

In Iran, President Hassan Rouhani said an agreement was close, but not quite done, describing the negotiations as "still steps away from reaching the intended peak."

Once an agreement is finalized, Congress will have 60 days to review it and hold a vote.

Sen. John Cornyn (R-Texas) sat down with Martha MacCallum this morning to discuss the possibility of lawmakers rejecting the deal. 

He said right now there is "broad bipartisan skepticism" about the details of the potential agreement, bringing up Israeli Prime Minister Benjamin Netanyahu's warning that a deal could move Iran closer to a nuclear weapon. 

"The devil is in the details," said Cornyn. 

MacCallum pointed out that the U.N. arms embargo on Iran could be lifted as part of the deal. 

Cornyn said that possible concession "seems crazy," blaming Iran for having its "fingerprints on every bad thing that's happening" in the Middle East. 

Watch the interview above.