The anticipated endorsement of Bernie Sanders as her preferred candidate for president by Rep. Alexandria Ocasio-Cortez on Saturday will not determine the winner of the Democratic presidential primary. Endorsements almost never matter. But this one does—for the future of the Democratic Party, not the future of its 2020 presidential campaign.

The timing of Ocasio-Cortez’s endorsement sends a sharp message to the party establishment and the progressive movement: We socialists are not here for Elizabeth Warren’s reformed capitalism. We socialists want socialism, and we’re not keeping quiet until we get it. Ocasio-Cortez seems to be telling us that Sanders and his movement will still be with us when he’s gone—and that she aims to be the one who leads it.

Ocasio-Cortez is making her move at a pivotal point in the race. Warren has eclipsed Sanders as not just the left-wing frontrunner, but the race’s frontrunner, leading or tied for the lead in most national and early state polls. Two weeks ago, Sanders suffered a heart attack, raising serious questions about whether his campaign could even continue. If Ocasio-Cortez didn’t see a significant difference between Warren and Sanders, or if she wanted to cozy up to Warren and be seen as part of her team, she would have great incentive to endorse Warren, urge her fellow socialists to join her, and help squelch any ideological skirmishing on the left.

Some progressive commentators worry that friction between Warren and Sanders—or more accurately, between Warren and Sanders supporters—is hurting the chances of progressives to sideline the party’s moderates and the donor class. “All you people squabbling at your desks about whether Warren or Sanders is scarier to big money are missing the damn point,” scolded Time editor-at-large Anand Giridharadas on Twitter. “Together they make up a two-headed populist uprising against plutocracy.” Late last month, The Nation’s editor, D.D. Guttenplan, advised progressive voters not to choose between Warren and Sanders until absolutely necessary. Instead, “hope the two candidates maintain their truce, competing to outdo each other in the boldness of their ideas and the breadth and passion of their support,” he wrote.

Yet Ocasio-Cortez is choosing, provocatively so. To make the endorsement at this moment is to say: Not so fast, Elizabeth.

Both Warren and Sanders want big tax hikes on the wealthy and corporations, single-payer health insurance, free public college, easier union organizing and tougher trade deals. Both have eschewed hitting the large donor circuit, stress that their agendas are designed to empower the poor and working class, and threaten the power of consolidated wealth. Both have scoffed at bipartisan incrementalism.

However, if you are a committed socialist, the differences between Sanders and Warren are crucial. Nathan Robinson of the influential socialist magazine Current Affairs last month detailed his problems with Warren. Some were rooted in tactical concerns that she wouldn’t build a political operation to overwhelm the ruling class, as she’s a “law professor” and not a “movement-builder.” But at bottom, Robinson wrote, “I don’t like to say that I can’t trust Elizabeth Warren, but I can’t.” He pointed to this year’s State of the Union address: “Donald Trump promised that America would ‘never be a socialist country.’ Warren stood up and applauded, as Bernie sat and fumed. This was a very clear ‘Which side are you on?’ moment.”

At a similar moment, Ocasio-Cortez has chosen to remind everyone of which side she’s on.

The differences are not strictly symbolic or theoretical. Take climate. Sanders envisions a truly massive federal government intervention to avert the climate crisis: a $16.3 trillion investment over 15 years, including the establishment of public ownership of electric utilities. Warren not only puts far less money on the table, but she also believes the profit motive can help protect the climate. Asked during the CNN climate forum whether she agreed with Sanders on public ownership, Warren stuck to her capitalistic guns. “Gosh, you know, I'm not sure that that's what gets you to the solution,” she said, and added, “If somebody wants to make a profit from building better solar panels and generating better battery storage, I'm not opposed to that.”

Warren has echoed Sanders on single-payer health insurance, even weathering question after question about how much she would raise taxes to pay for it. But the fact that she hasn’t always held that position, and still hasn’t released her own detailed plan, leads some to challenge the depth of her commitment. The socialist hosts of the Chapo Trap House podcast torched Warren in June for having a plan for everything but health care. Said one, “All of sudden all the specificity and granular policy … goes out the window when it comes to one issue that directly challenges a key constituency of the Democratic elite.”

Sure, they both propose taxing wealth, not just annual income. But Sanders is more aggressive, with higher rates levied on a broader class of multimillionaires, bringing $1.75 trillion more in revenue over 10 years. And he is more rhetorically aggressive. “I don't have a beef with billionaires,” said a somewhat charitable Warren in the last debate. Compare that with Sanders, who has said, “I don’t think that billionaires should exist.” Sanders has also acknowledged his wealth tax “does not eliminate billionaires” and that the day billionaires cease to be is “not tomorrow.” But it is clear he wants that day to come. His endgame is not her endgame.

Sanders put it plainly in a recent interview with ABC’s “This Week.” “Elizabeth, I think, as you know, has said that she is a capitalist through her bones,” he said. “I’m not.” (Her exact words were, “I am a capitalist to my bones” and she has also talked of her “love” of “what markets can do.”)

To some, this looks like semantics. The fact that Sanders does not have a platform of immediate government command over the entire economy has made some question not only the significance of his differences with Warren, but also why he even bothers to call himself a democratic socialist. Sanders even said in August: “The private sector is the sector of society which creates most of the wealth in America. And I want them to continue to do that” albeit checked and balanced by a “vibrant public sector.”

Still, by expressing a desire for government to own at least some of the means of production, and to place a ceiling on how much wealth individuals can accumulate, Sanders forces us to take his socialistic language seriously. Certainly, Ocasio-Cortez does.

Fox News’ Laura Ingraham views Ocasio-Cortez’s endorsement through the lens of crass political self-interest. “I keep thinking it’s really about a future presidential run for AOC,” Ingraham said. But if Ocasio-Cortez were primarily concerned with her own political ambition, the safer move would be to help the progressive with the best chance of winning the nomination get over the top, and be credited for helping to unify the party. An even safer move would be not to endorse at all.

The more obvious explanation is that Ocasio-Cortez wants to keep the socialist flame burning. Sanders is 78 years old and is in the twilight of his career. If the socialist movement that he has built is going to last, the torch must be passed. Ingraham may well be correct that Ocasio-Cortez wants to take that torch and eventually run for president herself. She would be just barely constitutionally eligible when she turns 35 in 2024. But her Sanders endorsement signals what kind of presidential campaign that would be—a direct descendant of the Sanders efforts that have put building a durable socialist movement in America above short-term electoral considerations.

That doesn’t mean Ocasio-Cortez will use the socialist torch to burn down the Democratic Party. She has said she will “support whoever the Democratic nominee is,” even if it’s Joe Biden, whom she politely criticized as someone who “does not particularly animate me.”

She told the New York Times last month of her time in the House so far, “I think I have more of a context of what it takes to do this job and survive on a day-to-day basis in a culture that is inherently hostile to people like me.” The Times noted she has “cut back on her appearances on behalf of Justice Democrats,” a left-wing organization that specializes in primary challenges, and replaced a publicly combative chief of staff with one who is “sober-minded.”

So Ocasio-Cortez is prepared to remain in the Democratic fold, but her endorsement of Sanders means she has no intention of shelving the socialist banner in the process. The Democratic Party’s big tent has long been filled with ideological tensions between moderates and progressives, but now there is also friction between progressives and socialists. And considering that Ocasio-Cortez is 48 years younger than Sanders, you can be sure that the socialist wing of the Democratic Party is not going away anytime soon.