Mr. Kerry cited Iran’s participation in the effort to bring about a cease-fire in Syria — one that critics view as a cynical effort to extend Mr. Assad’s rule — and the quickness with which Iran released American sailors after it seized them in the Persian Gulf in January as some of the positive results of the relationship.

But he also acknowledged that forces in Iran that “didn’t want the agreement in the first place” were doing all they could to stoke terrorism and press ahead on other ways to challenge the United States and Saudi Arabia, both of which are regularly denounced in Iran. Missile tests, cyberattacks and support of the militant group Hezbollah are not covered in the nuclear agreement.

It is a measure of the ugly tone, in both nations, that all talk of Mr. Kerry visiting Tehran before the end of Mr. Obama’s term is now dead. “It’s not even under discussion,” said Mr. Kerry, in a tone that contrasted sharply with his optimism a year ago, when the agreement was struck.

He added, “I’m not sure they would even want me to come at this moment,” because of the perception in Iran that the long-anticipated result of the deal — a freeing up of tens of billions of dollars in frozen assets, a flood of investment and an economic boom — was a false hope. He added, “I’ve got much bigger fish to fry than that right now,” a striking comment given the centrality of the Iran deal to Mr. Obama’s and Mr. Kerry’s legacies.

Few national security issues divided Washington like the nuclear deal, reached a year ago Thursday in Vienna. Every Republican in Congress who voted on the agreement opposed it. A year later, the battle continues. The House recently passed, overwhelmingly, an amendment to block a $17.6 billion deal for Boeing to sell aircraft to Iran Air. It would be the largest transaction between the United States and Iran since the hostage crisis 37 years ago.

Yet the congressional action has caused frustration in the aviation industry, because everyone agrees that if the Boeing deal falls through, Airbus will get Iran Air’s business. And it has become a symbol in Iran of an American effort to undermine the sanctions relief negotiated under the agreement.