Republican presidential front-runner Donald Trump faced a challenge at the Republican presidential TV debate in Michigan on Thursday as he failed to push back against his rivals’ charge that he’s unprepared to serve as commander-in-chief.

Towards the end of the debate, Senator Marco Rubio pointed to Trump’s lack of knowledge on foreign policy issues, thus questioning his preparedness to serve as commander-in-chief. “I think, as we’ve seen throughout this campaign, Donald has not shown a seriousness about the issues of foreign policy. He just simply hasn’t,” said Rubio. “The next president of the United States is going to have eight years of a mess of a foreign policy to clean up. That’s why it can’t be Hillary Clinton. And, quite frankly, that’s why it can’t be someone who simply has not shown the intellectual curiosity or the interest in learning about these very complicated issues.”

Instead of proving him wrong, Trump retorted to a personal attack on Rubio: “Let me just say this. I’ve gotten to know Marco over a period of time, believe me, he is not a leader. Believe me,” said Trump.

The notion that the Republican presidential front-runner lacks the experience and knowledge of foreign affairs prompted over 60 Republican foreign policy experts to express their opposition to a Donald Trump presidency. In a letter posted online, the national security experts wrote, “His vision of American influence and power in the world is wildly inconsistent and unmoored in principle. He swings from isolationism to military adventurism within the space of one sentence.”

“As committed and loyal Republicans, we are unable to support a Party ticket with Mr. Trump at its head. We commit ourselves to working energetically to prevent the election of someone so utterly unfitted to the office,” they pledged.

One of the signatories is Dov Zakheim, former undersecretary of defense in President George W. Bush’s administration, who also served as a top foreign policy advisor to Bush during the 2000 presidential campaign.

“Dealing in national security or national relations is very different from cutting a real estate deal. Even a foreign real estate deal,” Zakheim told Jewish Insider. “That’s number one. Secondly, his rhetoric has outraged friends, and allies, and partners around the world. I’ve just come back from overseas travel. People are either outraged or making jokes about it which is not very good for the United States either. Our European allies are worried because he talks about the changing alliances. Our Middle-Eastern allies are worried because the Israelis hear him talking about a degree of even-handedness that they don’t like. Our Arab friends don’t like the fact that he wants to keep Muslims out of the country. Our Japanese and Korean friends are concerned about his opposition to free trade. Our Central American friends and Mexican friends are concerned about his, what clearly appears to be a racist attitude. Wherever you go in the world, people are upset. How can a man like that be the leader of the free world?”

While the opposition to Trump’s candidacy is fierce and somewhat unprecedented, the Republican Party’s establishment has so far failed to unite behind one candidate that would be able to stop Trump from clinging to the nomination. “They’re going to have to figure out how they do this because if they keep dividing the rest of the vote, then Mr. Trump will get the nomination,” Zakheim stressed. “They have to decide who’s going to drop out. I think Carson did the right thing but you’ve got Mr. Kasich who’s never really done well except in one state. Now he hopes to win Ohio, but that’s not going to get him the nomination. You’ve got Mr. Rubio, hopes to win Florida, and he’s got Mr. Cruz who won Texas.”

“By themselves, it’s literally united we stand divided we fall. If they remain divided then they’ll fall to Trump,” he added.

Rubio, once seen as a clear alternative to Trump once the field winnows down, undermined his case on Thursday by pledging to support Trump if he becomes the nominee. Zakheim, however, said there’s no way he or any of his colleagues who signed the letter above would vote for Trump in the general election. “Let me put it this way, I don’t know whether I’ll vote for Hillary or whether I won’t vote at all but one thing I can guarantee, I can guarantee you I will not vote for Mr. Trump,” he told Jewish Insider.