Former Secretary of State John Kerry is under fire over a report claiming that he has been engaged in “shadow diplomacy” with officials from Iran and Europe as part of a final attempt to save the seemingly doomed 2015 Iran nuclear deal.

The Boston Globe reported Friday that Kerry sat down twice with Iranian Foreign Minister Javad Zarif in recent months to strategize in a bid to save the deal, as part of what the Globe described as “an aggressive yet stealthy” mission to put pressure on the Trump administration to keep the deal in some form.

Trump faces a May 12 deadline to review the deal, and Kerry has been ramping up his meetings ahead of that deadline. He reportedly met with German President Frank-Walter Steinmeier -- who was foreign minister of Germany when the deal was negotiated. He has also met with French President Emmanuel Macron twice and spoken by phone with European Union foreign affairs chief Federica Mogherini.

The Globe reported that Kerry was quiet in his campaign as he believes that a high-profile defense of the deal by prominent Democrats would only make Trump more likely to pull the U.S. out of the deal.

Kerry’s reported actions immediately sparked criticism and raised claims that such dealings with Iranian and European officials could violate the Logan Act -- which prohibits private citizens from negotiating on behalf of the U.S. government without authorization

While no one has ever been successfully prosecuted under the law, the Logan Act was raised last year over former national security adviser Michael Flynn’s negotiations at the United Nations over a resolution with Israel during the Trump transition.

“John Kerry helped craft a flaccid deal that granted the terror regime in Tehran vast monetary and geopolitical concessions, even as it put them on the glide path to nuclear weapons by its own terms,” Sohrab Ahmari, senior writer at Commentary magazine, told Fox News. “The biggest public service he can render now is to go away.”

Florida Sen. Marco Rubio said Kerry’s move “certainly raises Logan Act questions.”

He pointed to an alleged double standard, suggesting there would be a bigger outcry if Bush-era Secretary of State Condoleezza Rice had done the same during the Obama administration.

There was a significant outcry from Democrats and the Obama administration in 2015 when Republicans -- led by Arkansas Sen. Tom Cotton -- wrote to Iranian leadership explaining the difference between an executive agreement and a treaty.

The letter suggested that any deal without a congressional vote could be modified by a future Congress or revoked with “the stroke of a pen” by whoever replaces President Obama.

At that time, Kerry slammed Cotton's move as an “unconstitutional, un-thought-out action by somebody who has been in the United States Senate for 60-something days."

Harvard Law School professor emeritus Alan Dershowitz said Saturday that Kerry is lucky that no one has been prosecuted under the Logan Act or else he could be in trouble.

“Fortunately for everybody, the Logan Act [is a] dead letter but if it were in existence, my friend John Kerry would be violating the Logan Act,” Dershowitz said on “Fox & Friends.”

Although Kerry would not likely be prosecuted into the Logan Act, Dershowitz said that there are “real problems” with what Kerry is doing.

“He is negotiating, though he is not in the administration, and there are real problems with doing that,” he continued.

According to The Globe, Kerry has also been trying to get Congress on his side by placing dozens of phone calls, including to House Speaker Paul Ryan.

Kerry is reported to have coordinated his push with a group of former top State Department advisers who helped negotiate the Iran deal -- named Diplomacy Works. The group has reportedly chosen to focus on Europeans, Israelis and non-partisan experts to try and salvage the deal.

“This isn’t President Obama’s agreement. It’s the world’s agreement,” David Wade, a longtime Kerry adviser and advisor to Diplomacy Works, told The Globe. “Maybe Macron, Merkel, and Great Britain can persuade the administration, but if they can’t they’ll be even more essential to protecting the deal absent the United States. We know these voices are powerful. They have an audience with the president and our allies are popular at home.”

Diplomacy Works did not immediately respond to a request for comment from Fox News.

Trump has been critical not only of the Iran deal but also Kerry’s role in negotiating the deal in particular. He has repeatedly called him the “worst negotiator I’ve ever seen.”

On Friday, Trump mocked Kerry at the National Rifle Association’s annual conference in Dallas, Texas, describing him as “not the best negotiator we’ve ever seen” and mocking Kerry for breaking his leg during 2015 negotiations.

“He never walked away from the table, except to be in that bicycle race where he fell and broke his leg,” he said to cheers from the crowd. “That was the only time.”