WASHINGTON (Reuters) - As Congress faces a possible fight over the future of the Iran nuclear agreement, European ambassadors and officials from President Barack Obama’s administration are making their case for preserving the pact directly to U.S. lawmakers.

An Iranian flag flutters in front of the International Atomic Energy Agency (IAEA) headquarters in Vienna, Austria, January 15, 2016. REUTERS/Leonhard Foeger

The British, French, German and European Union ambassadors to the United States will participate later on Wednesday in a meeting on Capitol Hill with Democratic senators organized by the Senate’s number two Democrat, Richard Durbin, congressional aides and embassy officials told Reuters.

Former Undersecretary of State and lead Iran negotiator Wendy Sherman will also attend and former Energy Secretary Ernest Moniz and Treasury Secretary Jack Lew will participate via videolink, an aide to Durbin and another congressional aide said.

The meeting is part of an ongoing effort by Democrats in Congress and other officials who support the nuclear pact to bolster support for the deal by spelling out the consequences of its collapse as Republican President Donald Trump faces an Oct. 15 deadline for certifying the agreement or placing its fate in the hands of Congress.

A British embassy official said Ambassador Kim Darroch was in Congress on Wednesday with his French, German and EU counterparts meeting with both Democrats and Republicans “to provide information on the European position on the JCPOA,” using an acronym for the nuclear agreement.

An EU embassy spokesman confirmed that EU Ambassador David O’Sullivan and others would attend, to explain that the deal is a multilateral agreement that is working and that the European Union will do everything it can to ensure it stays in place.

Trump has long criticized the nuclear pact, a signature foreign policy achievement of his predecessor Obama, and signed in 2015 by the United States, Britain, France, Germany, Russia, China, the European Union and Iran.

Senior White House officials have said that Trump is leaning toward a course of action that could lead to the United States abandoning the pact, despite apparent disagreement within his administration over whether that is the best way forward.

A senior administration official said the administration was considering Oct. 12 for Trump to give a speech on Iran but no final decisions have been made.

Supporters of the deal say its collapse could trigger a regional arms race and worsen Middle East tensions. Opponents say it went too far in easing sanctions without requiring that Iran end its nuclear program permanently.

The ambassadors have said the deal’s demise would be a major loss that could lead to increased enrichment by Iran and weaken international proliferation efforts as the world grapples with a growing nuclear threat from North Korea.

Defense Secretary Jim Mattis said the United States should consider staying in the deal unless it were proven that Tehran was not abiding by the agreement.

Mattis said Iran was “fundamentally” in compliance with the agreement.

Earlier on Wednesday, Secretary of State Rex Tillerson said Trump would be presented with multiple options regarding the future of the nuclear pact.

Under the deal, Iran agreed to curb its nuclear program in exchange for the suspension of international sanctions that were choking its economy. If Trump declines to certify, it could pave the way for Congress to vote to resume those sanctions, killing the deal.

Some Republicans argue that Trump can decertify because he does not believe the agreement is in the national security interest. That, they said, would increase pressure on Tehran because Congress could threaten to re-impose sanctions if Iran does not agree to a more restrictive deal.

Iran has said it may abandon the nuclear deal it reached with the major world powers if the United States decides to withdraw from it.