“I don’t understand why it is that everybody’s working so hard to anticipate failure,” Mr. Obama said of the negotiations to hammer out a final nuclear accord, during a news conference at the end of a summit meeting of Latin American nations here. “My simple point is let’s wait and see what the deal is.”

He said he remained “absolutely positive” that if a final agreement codified the commitments agreed upon earlier this month, it would be the best way of preventing Iran from obtaining a nuclear weapon. And he said that he was worried that Republican critics of the deal were trying to “screw up” its completion.

Mr. Obama singled out Senator John McCain, Republican of Arizona, who he said had suggested that Secretary of State John Kerry was “somehow less trustworthy” than Iran’s supreme leader in describing the deal, calling it “an indication of the degree to which partisanship has crossed all boundaries.”

“That’s not how we’re supposed to run foreign policy,” Mr. Obama said. “That’s a problem. It needs to stop.”