Cory Booker would get to the subject of President Donald Trump and the resistance. He would get to the disunited Democratic Party and his vision for its future. But the New Jersey senator began the closing keynote of the Center for American Progress Ideas Conference last Tuesday with a story about Alice in Wonderland and his mother. He had recently watched her play the Red Queen in her retirement community’s production of the Lewis Carroll classic, and was moved by one of her character’s famous lines. After Alice declares, “one can’t believe impossible things,” the queen responds, “I daresay you haven’t had much practice then.... Why, sometimes I’ve believed in as many as six impossible things before breakfast.”

Booker had lots to say about the impossible on Tuesday. He quoted James Baldwin: “I know what I’m asking you is impossible, but in our time, as in every time, the impossible is the least we can demand.” He spoke of “an impossible dream in America that has yet to be made real.” It was an earnest and emotional speech about finding light in a time of darkness, and it set Booker apart from several other senators—and potential 2020 presidential candidates—who gave speeches that day. Kirsten Gillibrand and Kamala Harris each gave shorter remarks, focused on their pet issues of paid leave and criminal justice reform, respectively. Elizabeth Warren delivered the lunch keynote, railing righteously, as she always does, against concentrated money and power in politics. None of them held the crowd’s attention like Booker.

#CAPIdeas has already heard today from Klobuchar, Murphy, Warren, Gillibrand, Harris. Only @CoryBooker has spellbound the crowd. — Ed O'Keefe (@edatpost) May 16, 2017

Booker’s a fantastic orator who trades Obama’s coolness for we’re-all-in-danger angst. Barack Emo-bama. — Dave Weigel (@daveweigel) May 16, 2017

Booker’s rousing rhetoric is a key reason he’s considered a 2020 contender, but his crowd-pleasing performance on Tuesday belied the hostility he’d face in a Democratic primary. A neoliberal who’s cozy with the monied elites of Wall Street and Silicon Valley, he’s distrusted by many on the left. “He’s a non-starter right now,” Markos Moulitsas, founder of the influential liberal website Daily Kos, told me. “He hasn’t proven his ability to distance himself from the Wall Street and Big Pharma interests that have basically been the bedrock of his support.”

Booker’s defenders have long refuted this criticism. He has taken on the financial industry in the Senate, most recently by opposing the attempted rollback of Wall Street regulations and an Obama Labor Department retirement-savings rule finalized last year. Nor is Booker as centrist as New York Governor Andrew Cuomo, another blue-state 2020 prospect. But in this populist moment, as some progressives demand ideological purity from Democrats, Moulitsas’s widely held criticism might be enough to sink Booker’s chances in 2020.

There’s also the problem of Booker’s brand, which is perhaps best described as positivity politics: He embraces bipartisanship, and refuses to vilify his political opponents. His warm and generous spirit would be welcome under normal political circumstances, but his style has lost currency under a unified Republican government, as the Democratic base demands outright obstruction. Booker has shown of late, albeit haltingly, that he can move left with the times. But could he be convincing as a more combative progressive, or would his reputation overwhelm his rhetoric? More importantly, would doing so erase what makes Booker unique—the very qualities that enabled his swift ascent to national stardom, and which no doubt would serve him well in a general election against Donald Trump?