French President Emmanuel Macron, German Chancellor Angela Merkel and British Prime Minister Theresa May have all done their best to convince US President Donald Trump that his beef with the Iran nuclear deal is misguided and that the agreement limiting Tehran's potential pursuit of nuclear weapons, while flawed, should be preserved, while other concerns are addressed.

Their appeals have had no visible effect but the outcome will be known on Tuesday, ahead of the official May 12 deadline for the US to declare whether it would continue to waive sanctions against Iran, and thus remain one of the six parties to the 2015 agreement formally called the Joint Comprehensive Plan of Action (JCPOA).

The chairman of the European Parliament's foreign affairs committee, German lawmaker David McAllister, summed up Europe's frustration, fears and final efforts to save the accord. "I'm trying to understand the US position," McAllister said with clear exasperation at an event organized by the advocacy group United Against Nuclear Iran (UANI). "This agreement doesn't belong to a single country. We in Brussels say this is a multilateral agreement, endorsed by a United Nations Security Resolution. We believe it cannot really be changed unilaterally because it belongs to us all."

McAllister said European lawmakers were personally lobbying the US Congress, explaining that "every colleague who knows a colleague on the Hill" was calling them, "to explain what is actually at stake."

Roule: Tehran should be the target

But last-ditch efforts are probably not enough, according to Norman Roule, who spent 34 years in the Central Intelligence Agency, many of them focused on Iran. Speaking with DW after the UANI event, Roule suggested European leaders perhaps should have spent their time pressuring Tehran, not Trump.

The EU, said Roule, should have made clear it would enact further sanctions on Iran swiftly over its ballistic missile program and "regional adventurism." "These complaints have been known and widely talked about for two-and-a-half years. So to simply say that because the world hasn't done anything about it, it is the problem of the Trump administration is not quite fair." He cited Israel's recent claims that it has proof of Iran's intent to acquire nuclear weapons as impetus to abandon this agreement in favor of stronger restrictions. The EU argues the opposite. 

Roule said US and European diplomats had been meeting "quietly and out of the press" on these issues since last summer but that to forestall the US move, he believes the EU would need to have come up with a program demonstrating it could extract real concessions from the Iranian government, not just "actions that could dissuade the United States from undertaking its withdrawal from the deal. I think in the end it's going to be hard to convince Iran to accept these changes," he said, "and right now there are not sufficient pressures against the Iranian government."

Schaake: time for GOP to stand up

While Roule says there are many people other than Trump who support ditching the JCPOA, Dutch MEP Marietje Schaake has a different view. Schaake is vice-chair of the EP's delegation for relations with the US and has been watching Congress closely. "There's actually a lot of people in Washington who are as worried about what the president may do as we are," Schaake told DW. "The most important pushback should come not only from Congress but from the Republican Party. This may well be a moment of truth for them to look at what kinds of checks and balances they're willing to provide, when the moment is that they're going to push back against a president that is ripping up deals and being unpredictable with potentially devastating consequences."

Read more: Opinion: John Bolton's rise could be death knell for Iran deal

Schaake said if Trump truly wanted more international condemnation of Iran's behavior, threatening to tear up the nuclear agreement was the wrong way to do it. "It has taken away time, energy, attention from other topics that we could have been focusing on such as the toxic role of Iran in the region, such as human rights, so I think it is really a bad approach by the White House." She said she was still hopeful he would change his mind about quitting the JCPOA, but "if not I hope the international community will stay as closely together as possible to limit the fallout."

Read more: What is the Iran nuclear deal?