President Barack Obama speaking on Thursday about the breakthrough in the Iranian nuclear talks. (Pablo Martinez Monsivais/Associated Press)

PRESIDENT OBAMA has launched an aggressive lobbying campaign for his preliminary nuclear deal with Iran, telling the New York Times on Saturday that “this is our best bet by far to make sure Iran doesn’t get a nuclear weapon.” Unfortunately for those who disagree, the president’s claims could have a self-fulfilling quality. By loudly insisting there is no alternative to the terms he has agreed to, Mr. Obama helps ensure that the option of maintaining sanctions while insisting that Iran agree to dismantle more of its nuclear infrastructure — as Israel and some in Congress advocate — is no longer a practical alternative.

At the same time, the White House stance risks weakening its negotiating position in the crucial bargaining with Iran. Mr. Obama conceded that some vital “details” have not yet been worked out, including how the lifting of sanctions on Iran would be tied to its implementation of steps such as reducing its stockpile of enriched uranium and its base of installed centrifuges. Worryingly, the fact sheets issued by the U.S. and Iranian governments differ sharply on this point: While the U.S. version says that “sanctions will be suspended after” the International Atomic Energy Agency “has verified that Iran has taken all of its key nuclear-related steps,” Tehran’s account is that “at the same time as the start of Iran’s nuclear-related implementation work, all of the sanctions will be automatically annulled on a single specified day.”

The gulf between those two scenarios is extremely important. Unless sanctions relief is conditioned on Iranian performance, the United States and its partners will lose their leverage.

Similarly crucial differences may be buried in the as-yet-unspecified details of how inspectors will obtain access to new suspected nuclear sites and how they will get answers to outstanding questions about Iran’s previous work on nuclear warhead designs. By insisting that the deal is already the best available, Mr. Obama is making it more difficult for his negotiators to walk away from the follow-up talks if they are unable to obtain satisfactory terms. If Iran continues to insist on the “immediate” lifting of sanctions, will the president give in rather than reverse his rhetoric?

The White House push to sell the deal is in part a reaction to the possibility of congressional action later this month. A bipartisan bill would block Mr. Obama from implementing a deal for 60 days after its conclusion and would require a vote to move forward; Sen. Bob Corker (R-Tenn.), one of its sponsors, said Sunday that it was within two or three votes of veto-proof support. But Mr. Obama would not have to reckon with that authorizing vote until after the accord was completed. As Mr. Corker argued, if it is really the “good deal” that Mr. Obama describes, “surely he can sell this to the United States Senate and the House.”

The White House is threatening to veto Mr. Corker’s bill. But Mr. Obama sounded a conciliatory note in the Times interview, saying “my hope is that we can find something that allows Congress to express itself.” There’s room for a compromise there, and, as Mr. Corker noted, the prospect of an eventual vote could help Mr. Obama extract the concessions still needed from Iran.