President Barack Obama aggressively pushed back against Republican criticism of a deal his Administration helped negotiate to keep nuclear weapons out of Iran’s hands, telling reporters traveling with him to Ethiopia that the GOP presidential hopefuls who hope to succeed him were using “ridiculous” and “ad hominem” attacks to avoid a serious debate rooted in substance.

During a news conference in Addis Ababa, Obama was asked about remarks made a day earlier by Mike Huckabee, the former Arkansas Governor who compared the deal to the Holocaust. Obama used the question to level broader criticism of the jockeying happening inside the Republican Party that is still in the early days of a search for a 2016 nominee.

“I have not yet heard a factual argument on the other side that holds up to scrutiny,” Obama said of his GOP critics. “There is a reason why 99 percent of the world thinks this is a good deal. It’s because it’s a good deal.”

The Republican candidates looking to follow him into the White House do not share his analysis. For instance, Huckabee said the proposed deal gives too much trust to Iran and betrays Israel, musing to the conservative Breitbart News that Obama was ready to “take the Israelis and march them to the door of the oven.”

The oven remark was a clear reference to the Holocaust, when Nazis murdered millions of European Jews in concentration camps. Iranian leaders often use strong anti-Israel rhetoric, promising “death to Israel,” though few analysts think the country is actually preparing to engage in a genocide.

Obama, whose great uncle helped to liberate part of the Buchenwald camp in Germany, took offense to Huckabee’s rhetoric and said it matched up with other statements he has heard from Republicans. Obama said it “would be considered ridiculous if it weren’t so sad.”

“We’re creating a culture that is not conducive to good policy or good politics. The American people deserve better. Certainly, presidential debates deserve better,” Obama said, speaking broadly about a crowded Republican field that has been marked by daily oneupmanship. “In 18 months, I’m turning over the keys. I want to make sure I’m turning over the keys to somebody who is serious about the serious problems that the country faces and the world faces.”

Obama has long shown an impatience with the political showmanship, especially when it comes to foreign policy. The President said his would-be-replacements should take time to more carefully study the issues before jumping forward with criticism to motivate the party’s base.

“We have robust debates. We look at the facts. There are going to be disagreements but we just don’t fling out ad hominem attacks like that because it doesn’t help inform the American people,” Obama chided.

Speaking in Des Moines, Democratic frontrunner Hillary Clinton joined Obama in criticizing Huckabee’s rhetoric.

“Comments like these are offensive and have no place in our political dialogue,” Clinton said. “I am disappointed and I’m really offended personally.” The former Secretary of State said there is space for candidates to disagree on the details of the deal, but said Huckabee’s rhetoric “steps over the line” and deserved to be repudiated.

For his part, Huckabee was not backing off. From his campaign headquarters in Little Rock, Ark., Huckabee released a statement that doubled-down on his earlier rhetoric after hearing Obama’s criticism.

“What’s ridiculous and sad is that President Obama does not take the Iran threats seriously. For decades, Iranian leaders have pledged to ‘destroy,’ ‘annihilate,’ and ‘wipe Israel off the map’ with a ‘big Holocaust,’” Huckabee said. His statement to reporters included links to Iranian leaders’ comments using that rhetoric.

Huckabee pledged he would never allow that to happen. “I will stand with our ally Israel to prevent the terrorists in Tehran from achieving their own stated goal of another Holocaust,” he added. In a follow-up email to supporters, Huckabee asked them to sign a petition urging Congress to skip its summer break and stay in Washington to “fulfill your constitutional duty and KILL the dangerous Obama-Kerry nuclear deal with the Iranians.”

There was no real downside for Huckabee to continue his line of criticism. The former Baptist pastor is a favorite among the evangelical wing of the GOP and he is a frequent guide to the region, taking paying guests on tours of Biblical sites. Republicans—especially those voters who pick the party’s nominee—overwhelmingly support Israel and are more than willing to listen to criticism of anything Obama backs.

Republicans have been almost unified in their opposition to the deal with Iran, which was negotiated by world powers China, France, Russia, the United Kingdom, Germany and the United States.

Under the deal, Iran will get relief from sanctions and regain access to international oil markets, which will bring it a windfall of about $100 billion. In exchange, Iran must dispose of most of its low-enriched uranium, stop efforts to produce or acquire more nuclear fuel and consent to inspections. The deal is expected to block Iran from obtaining the technology needed to build a nuclear weapon for at least a decade.

With additional reporting by Sam Frizell.

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Write to Philip Elliott at philip.elliott@time.com.