BRUSSELS — Having apparently lost the fight to dissuade President Trump from decertifying the Iran nuclear deal, the European countries that signed the pact are now looking to the United States Congress as a last chance to preserve the treaty and some shred of Western unity.

Mr. Trump, who has described the deal as “disastrous” and “the worst deal ever,” must decide by mid-October whether to certify that Iran is in compliance with the nuclear agreement — something he has done only reluctantly twice before.

Reports from Washington suggest that Mr. Trump will in fact decertify the deal, overruling his own national security team, but at the same time quietly encourage Congress not to reimpose punitive economic sanctions on Iran that were lifted under the accord.

European diplomats say the European nations that signed the accord are hoping to use the same arguments that failed to sway the president as they press American lawmakers to preserve an agreement that they see as essential to maintaining regional stability. They contend that the North Korean nuclear crisis is difficult enough without starting a new one unnecessarily, that American credibility is at stake and that Western division over Iran can only benefit Russia.