The Simon Wiesenthal Center has called on congressional leaders to roundly reject the “gutter politics” of dual loyalty accusations directed against senators who oppose the nuclear deal with Iran, and urged US leaders to counter such rhetoric.

In a statement released Wednesday, the dean of the Jewish organization, Marvin Hier, and associate dean Abraham Cooper denounced the abuse directed at the Democratic senator Charles Schumer, who declared last week that he would vote against the nuclear deal with Iran deal in an upcoming congressional vote.

“The spectacle of labeling Senator Schumer and other opponents of the controversial Iran nuclear deal as ‘warmongers’ who are more loyal to Israel than America is the lowest form of gutter politics seen in our country since Joe McCarthy,” they said.

“Instead of passionate and reasoned debates based on objective analysis of facts, we are increasingly witness to eblasts, political cartoons, and sound bites that outrageously call into question people’s loyalty to our nation,” Hier and Coope’s statement said. “We fear that such hateful rhetoric between now and next month’s historic vote will legitimize mainstream hate and anti-Semitism and falsely reduce an important policy decision that impacts on international terrorism, our Arab allies and the future of the Middle East, to a disagreement between the US and Israel.”

They called on congressional leaders to start “pushing back against the dangerous rhetoric that has the potential to wreak more damage to our society, than any foreign threat.”

The dual loyalty charge came to the fore after Schumer, the highest-ranking Jewish Democrat in the Senate, announced that he was opposed to the deal, reached on July 14 between Iran and six world powers. A weekly cartoon on Daily Kos, a liberal website, depicted Schumer as a woodchuck, and in the course of a TV interview, the flag in the woodchuck’s office changes from American to Israeli and the moderator, a basset hound, calls Schumer a traitor. The cartoonist, Eric Lewis, has had drawings published in the New Yorker.

The cartoon drew outraged responses.

“There is room for a legitimate debate on the Iran deal; however charges” of disloyalty “against Senator Schumer — and any other members who articulate on fact-based but alternative views — are beyond inappropriate,” Jonathan Greenblatt, the new national director of the Anti-Defamation League, told The Times of Israel.

With most Republicans against the deal, Democrats have become the battleground — and Schumer has been under especially intense scrutiny. Congress has until late September to decide whether to accept the agreement.

The Democratic caucus generally defers to those within the party with the biggest stake in an issue, and traditionally has looked to its Jewish caucus, some 27 members, for leadership on Israel-related issues. Six have declared against the deal and 10 have declared for it. But Schumer’s coming out in opposition was seen as a watershed because he is in line to succeed Senator Harry Reid, the party’s leader in the Senate who is retiring next year.

In this July 16, 2015 file photo, Sen. Charles Schumer, D-N.Y., speaks during a hearing on Capitol Hill in Washington, Thursday. (AP/Susan Walsh)

Online, some Obama critics lost no time in drawing a line between the Daily Kos cartoon and Obama’s rhetoric defending the deal.

“The president’s dog whistles are heard by the president’s dogs,” David Frum, a former speechwriter for President George W. Bush and now a senior editor at The Atlantic, tweeted.

A number of conservative commentators had already said that Obama was insinuating anti-Semitic tropes about dual loyalty in addresses he has delivered defending the deal.