Bernie Sanders had a big test at the first Democratic debate: to prove that he could broaden his appeal enough to be a genuine threat to Hillary Clinton for the nomination. While the Vermont senator has made recent gains in the early states of Iowa and New Hampshire, he’s still more than 20 points behind her on average—and that’s with Joe Biden listed as an option and drawing support from Clinton. After Sanders' poll numbers rose sharply in the summer, he's plateaued, stuck in the mid-20s. That raises the question of how much higher he can really go, especially with Clinton's sizable advantages among women and voters of color. Though he had some memorable moments that lit up Twitter on Tuesday, the debate revealed why the political instincts that have fired the progressive base could ultimately keep him from breaking through to the rest of the party. Clearly, Bernie's gonna be Bernie. 

The debate showed why his fans are so passionate about him: Sanders is at his best when he’s going on the offensive—against the big banks, Wall Street, and the “millionaires and billionaires” who are funding elections. But the debate also forced him to go on the defensive, pushing him out of his comfort zone, and on those occasions, Sanders simply muddled through. Those responses spoke to his limitations as a candidate whose greatest strength and greatest weakness is his singular focus on anti-corporate economic populism. 

Even before the most recent mass shooting in Oregon, it was clear that gun control would be Sanders’s biggest vulnerability, as it’s one of the few issues where he’s consistently been more conservative than Clinton. But his response to CNN moderator Anderson Cooper—who pointed out his vote against the Brady bill to mandate background checks—showed none of the ideological clarity that undergirds his economic vision. He started off with an awkward reference to himself in the third person, citing his D-minus rating from the NRA. But his main fallback was his parochialism, repeatedly citing the fact that he comes from a “rural state” with different views on gun control rather than providing any ideological explanation or argument for his pro-gun votes. When Martin O’Malley tried to push him on the issue, saying it “was not about rural and urban,” Sanders stubbornly repeated the same talking points: “It’s exactly about rural,” he said, then trotted out the line about his “D minus record” from the NRA again. 

Sanders repeated the same pattern on other issues, providing patchy answers to questions outside of his purview. Perhaps his most underwhelming moment during the debate was the discussion on foreign policy, which rarely features in his big speeches. When he was asked to weigh in on former Senator Jim Webb’s vendetta against China, he seemed to be caught unawares. “Pardon me?” he said. After CNN moderator Anderson Cooper repeated the question, he launched into an argument that Putin, somehow, would end up regretting his decision to send Russian troops into Syria.  

Cooper nearly laughed, it seemed, responding: “He doesn’t seem to be the type of guy to regret anything,” he said. But Sanders continued pursuing the same logic, arguing that Putin was already regretting his decision to intervene in Ukraine and Crimea. That would ultimately lead Putin to come around, he said, because “the Russian people are going to give him a message,” without explaining how a leader who’s been so intractable would end up changing his tune and suddenly be willing to collaborate on a non-military solution with the U.S. 

Sanders didn’t get to explaining how he would lead the U.S. effort differently in Syria until later in the debate when he asked point blank about the issue. It was a prime opportunity for him to slam Clinton for her hawkish foreign policy views; she’s far more eager than President Obama to use military force in places like Syria. But Sanders largely declined to take the opening, despite having put out a press release this weekend spotlighting his anti-war views. At the debate, he laid out his policy positions, denouncing the war in Iraq as “the worst blunder in the history of the country” and opposing ground troops in Syria; he delivered a passing shot at Clinton's support of a no-fly zone over Syria. But he mostly avoided using his dovish positions to tackle Clinton’s hawkishness head on, even though it’s one of the areas where the next president will matter the most, as my colleague Jeet Heer explained. Clinton, by contrast, took a direct jab at Sanders: After saying he did not support ground trips in Syria, she swiftly interrupted the moderator to interject, "Nobody does, Senator Sanders."

Those moments in the debate seem to reflect the fact that Sanders is still a candidate who views politics through a single prism—and doesn’t care as much about what falls outside of it. That disinterest was evident even before the debate began: He barely spent any time preparing, and his campaign aides essentially had to push him into it. The one issue on which Sanders has been forced to stretch himself during the campaign has been on criminal justice reform and racial inequality, but it took major public missteps before he began talking about the issues outside of the economic inequality lens. He had better answers to the questions raised by the Black Lives Matter this time—but he'd been forced many times before to prepare for them. 

Essentially, the debate seemed to confirm that Sanders isn’t interested in stretching himself as a candidate for the sake of broader political appeal. That’s exactly why his supporters love him so much: He is a true believer who believes that “the casino capitalist process” has corrupted the country and is unwavering in his focus on the issue. And that message is having an undeniable impact on the Democratic primary. This debate opened with a discussion between Sanders and Clinton of “Democratic Socialism," and the merits of capitalism—subjects that would never have arisen without Sanders in the field. But his moments of weakness in the debate also suggested that he doesn't have the political instincts or the appetite to court voters on issues outside of those policy priorities. And Bernie, being Bernie, would probably consider that a compliment.