In this contentious Republican presidential primary, with candidates constantly hurling insults at each other, there was one island of calm. Ted Cruz and Donald Trump had a mutual admiration society that verged on a bromance. 

While other candidates often balked at Donald Trump’s rude assaults and questioned whether he even belonged in the Republican Party, Cruz was a constant defender. But now even Cruz and Trump have had a falling-out. With Trump suggesting that Cruz’s birth in Canada might make his presidential bid vulnerable to a constitutional challenge, Cruz responded with a tweet that implied that the former reality TV star had finally “jumped the shark.”

Cruz’s tweet wasn’t exactly a blunt rebuke. It was a link to a YouTube clip of the infamous “jump the shark” scene from Happy Days, an allusion that requires pop-cultural knowledge and a measure of interpretation. Still, this was as close to angry words as Cruz has so far been able to muster towards Trump. What’s remarkable is how much Cruz has tolerated over the last six months when the two were de facto allies, revealing how cynical and self-interested Cruz is capable of being. And their break-up, in turn, reveals the extent of Cruz’s isolation in the Republican Party.

Hitherto, Cruz has been Trump’s most stalwart defender among his rivals, often speaking of the real estate mogul in terms that sounded as if he were auditioning to be his running mate, rather than running against him. The political rationale behind Cruz’s praise of Trump has been obvious: They are both grazing in the same part of the electorate, among Republicans who are unhappy with the establishment. Cruz has clearly been hoping that Trump’s supporters will eventually turn to Cruz when they decide that Trump is not a viable candidate. 

So time and again, Cruz stood by Trump when others blanched. When Trump launched his campaign by attacking Mexican undocumented immigrants as rapists and drug dealers, Cruz was there to defend him. “I like Donald Trump. I think he’s terrific, I think he’s brash, I think he speaks the truth,” Cruz told Fox News. That earned a grateful tweet from Trump. 

Many Republicans were appalled when Trump trampled on John McCain’s war record, saying, “He’s not a war hero. ... He’s a war hero because he was captured. ... I like people that weren’t captured.” But Cruz was the rare Republican who avoided condemning Trump. “I recognize that folks in the press love to see Republican-on-Republican violence, so you want me to say something bad about Donald Trump or bad about John McCain or bad about anyone else,” Cruz said. “I’m not going to do it. John McCain is a friend of mine. I respect and admire him and he’s an American hero. And Donald Trump is a friend of mine.”

Nor did Cruz have any condemnation for Trump’s incendiary call to ban Muslims from entering the United States. In mild terms, Cruz disagreed with Trump’s proposal, saying“I do not think it is the right solution.” But in a press conference in December, Cruz made clear that this dispute over policy didn’t impinge on his high regard for Trump, stating, “I do not believe the world needs my voice added to that chorus of critics. ... And listen, I commend Donald Trump for standing up and focusing America’s attention on the need to secure our borders.”

So Cruz was willing to tolerate Trump’s insults to Mexicans and Muslims as well as stalwart Republicans like John McCain. It was only when Cruz himself became the subject of Trumpian fire that he decided enough was enough. Cruz’s position is cowardly and self-centered: he refused to have stronger words for Trump’s outrages because he didn’t want to offend Trump’s supporters, and that pusillanimity was in the service of an extremely selfish worldview. 

Cruz can be compared to Republicans like Dick Cheney and Rob Portman who came out in favor of marriage equality because they had gay relatives. While their change of heart is commendable, the fact that it came about only because they were personally touched by the matter is telling. It points to a fundamental lack of empathy, in that they couldn’t change their view on the matter until a close family member was affected. Similarly, Cruz has no empathy for Mexican or Muslim immigrants, or even a trace of fellow-feeling for a fellow Republican senator like John McCain. It’s only when Cruz himself was insulted that he stood up to Trump, because Cruz has no greater principle than his own self-interest.

Having broken with Trump, Cruz finds himself almost friendless in the Republican Party. And now that Trump has made Cruz birtherism an issue, many others are eager to join in the pile-on. In a radio interview yesterday, McCain, who had to deal with a similar issue in 2008, said the question of whether Cruz is eligible to be president is “worth looking into. I don’t think it’s illegitimate to look into it.” 

Trump questioned McCain’s heroic war record, but McCain still hates Cruz much more (even if this is because of Cruz’s right-wing grandstanding in the Senate rather than Cruz’s alliance with Trump). Whatever the cause, Cruz has many enemies in the Republican Party who are more than willing to help Trump demolish the Texas senator.