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.