As to Iran’s nuclear obligations under the agreement, Mr. Trump asserted that “we got weak inspections in exchange for no more than a purely short-term and temporary delay in Iran’s path to a bomb.” This will be news to the I.A.E.A., which now benefits from the most intrusive and transparent inspection and monitoring regime ever devised. Its major provisions last 20 to 25 years, and under the Additional Protocol that Iran is obliged to adopt under the agreement, the I.A.E.A. will have the right to access declared and undeclared sites in perpetuity, including Iran’s military facilities.

And that “short term” delay in Iran’s race to a bomb? Mr. Trump ignored the fact that while certain constraints on Iran’s enrichment capacity go away in 10 or 15 years, it is permanently barred under the Non-Proliferation Treaty from acquiring a nuclear weapon or doing weapons-related work. If Iran nonetheless moves in that direction, thanks to the inspection regime a future president will have a much greater likelihood of seeing it and a united international community to confront it — both of which the United States will lose if it reneges on the deal.

Now that Mr. Trump has decertified Iran’s compliance with the nuclear agreement, Congress has 60 days to decide whether to reimpose sanctions. Mr. Trump called on Congress and America’s allies to use the time “to address the deal’s many serious flaws.” If not, he said, “the agreement will be terminated.”

By “fix” Mr. Trump means legislation to impose new conditions on Iran beyond the purview of the agreement and to extend its constraints indefinitely. That would put the United States, not Iran, in violation of the agreement and isolate Washington, not Tehran, around the world. It would allow Iran to resume its pursuit of nuclear weapons or to stick with the deal for its economic benefits, forcing the United States to sanction its closest allies for doing business with Tehran. It would provide a “we told you so” gift to Iranian hard-liners in their struggle with pragmatists. It would shackle, not advance, Mr. Trump’s ability to sign others on to his broader strategy to confront Iranian aggression. More broadly, it would undermine America’s credibility — and its ability to strike agreements that make the country safer in the future.

Congress must resist the temptation — and the political pressure — to unilaterally renegotiate the Iran deal and therefore kill it. Instead, it could usefully lay out what Mr. Trump’s speech did not: an actual comprehensive strategy to contend with Iran’s non-nuclear behavior, including diplomatic efforts to end the conflicts in Yemen, Syria and Iraq that Iran exploits; stronger security cooperation with the Gulf States and Israel; better coordination with America’s allies; and targeted sanctions on Iran that do not violate the nuclear accord. Unlike Mr. Trump’s decision to decertify Iran, that would be a real contribution to America’s security.