Like the title character of Samuel Beckett’s Waiting For Godot, Bernie Sanders’s endorsement of Hillary Clinton has been so long expected and announced that it felt like it would never show up. But unlike Godot, the endorsement is here. Hillary Clinton, Sanders acknowledged at a joint rally Tuesday morning in New Hampshire, “will be Democratic nominee, and I intend to do everything I can to make certain she will be next president of the United States.” He sounded like he meant it, even if Sanders came off as more fervently anti-Trump than pro-Clinton. 

Clinton clinched the nomination more than a month ago, and the time it took Sanders to endorse her has been a source of vexation to many party stalwarts. Last week, House Democrats even booed their Senate colleague at a meeting for drawing out the end of his campaign longer than many deemed necessary. 

Yet from another perspective, Democrats should be impressed that Sanders is joining their ranks at all, considering that his political identity for decades has been as someone who was left-of-center but proudly, pointedly independent. Throughout his political career, Sanders has preferred to identify himself with entities like the Socialist Party of America or the Liberty Union Party rather than the Democratic Party proper. Even after announcing he’d become a Democrat last November—a necessary step for ballot access in some states, and for courting, you know, Democratic primary votershe’s still listed as an independent in the Senate.

If Sanders has had a consistent philosophy over the course of his career, it’s that effective change has to come from outside the Democratic Party. But now, in what is both the peak and twilight of his long career, Sanders has no real choice: he has to wholeheartedly embrace a leadership role in the very party he so long resisted joining. To fail to take up that leadership role would be to abandon his formidable legacy of reviving economic populism in the Democratic Party.

Strangely, it was only in losing the nomination to Hillary Clinton that Sanders developed a real institutional stake in the Democratic Party. Clinton and the party have now adopted key tenets of Sanders’s message, at least in modified form, on a host of crucial issues: the Keystone pipeline, the minimum wage, college affordability, trade deals, taxing climate emissions. Sanders hasn’t gotten everything he wanted, of course—primary losers never do—but enough that he can plausibly proclaim“We now have the most progressive platform in the history of the Democratic Party.”

But with a newly progressive platform comes a different responsibility for the man who did so much to bring it about. Sanders can no longer afford to continue casting himself an outsider criticizing the corruption of the system. As the historian Michael Kazin of Georgetown University told The Washington Post, “social movements succeed when they get some portion of the political elites on their side.” And Bernie Sanders, like it or not, is now a certified political elite. 

The industrial unions of the 1930s needed to have Robert Wagner in the Senate and Frances Perkins in the Department of Labor; the civil rights movement of the 1960s needed Lyndon Johnson’s political muscle. For Sanders’s “revolution” to succeed, his policies need a watchdog in Washington, a powerful person to guard them from attack and shepherd them into being. And who could that watchdog be but Bernie Sanders? There are other stalwart populist Democrats in Washington, notably Elizabeth Warren and Sherrod Brown, but no one else who has led a national movement with millions of voters from around the country who have signed up for a progressive agenda and, in many cases, donated money. Sanders’s email list alone gives him a formidable way to mobilize popular discontent and hold the Clinton administration accountable. An email from Bernie could cause a flood of protests or letters of support, depending on how the administration behaves.  

Bernie has resisted the system throughout his political career, but now he has a stake in it. Historically, it’s been very easy for insurgent campaigns to dissipate after defeat. But from time to time, a political movement has longer staying power because it gains a foothold in elite institutions. The defeated insurgency of Eugene McCarthy in 1968 galvanized opposition to the Vietnam War that continued to fester in Congress, ultimately leading to the creation of the War Powers Act and other checks on presidential power. But that was only possible because McCarthy and others continued to push for anti-war sentiment in the halls of power and rally their followers around it even after his defeat. For Sanders’s agenda to have a similar life, he has to become a Washington insider, and start lobbying the party he has reluctantly joined.