After losing the New York primary on Tuesday, Bernie Sanders went home to Vermont to "recharge." While he's not about to fold up his presidential campaign (nor should he), it has become clear that barring some catastrophe, Hillary Clinton is going to be the Democratic nominee. So one imagines Sanders taking a long walk in the woods, asking himself some profound questions about his future. Where do I go from here? What do I want? And how can I get it?

He's back on the trail now, and there's no telling whether he answered those questions for himself, or if he even asked them. But if he doesn't come up with the right answers, he could squander the chance to shape the next presidency, and the party he recently adopted, after a lifetime as a political independent.

When these questions come up, we usually think about them in abstract terms, like whether Sanders can "expand the debate" and "pull Democrats to the left" (I've written these things myself). But when you try to turn those ideas into concrete things Sanders could do, exercising influence becomes more challenging. Unlike someone like Ronald Reagan, who pulled Republicans to the right so he could eventually win his party's nomination after trying and failing, this will almost certainly be Sanders' last run for president. So what can he do in the short and long run?

His first chance to demonstrate the effect he's had on the party will come at the convention, where Sanders will demand the party's platform reflects his agenda. He might get some Sanders-esque (Sandersian? Bernificent?) language inserted, but truthfully, the platform is almost entirely symbolic, and most people never read it (here's the Democrats' 2012 version, if you've got some time to kill). It's mostly vague statements of principle, and if there were something in there Clinton wanted to ignore during her presidency, she could probably do it without much risk.

Then there's the question of whom Clinton will appoint to key positions that Sanders cares about. The president makes thousands of appointments, and as they say, "personnel is policy." I don't know if Sanders has lists of people he'd like to put in places like the Securities and Exchange Commission, but something tells me he hasn't gotten that granular yet. But it would be much more effective to be in on that process early, rather than waiting to object to some Wall Street figure Clinton wants to install as a key regulator.

Which brings up another challenge for Sanders: specificity. He has been criticized for his policy proposals being thin on details. If he wants to exert real influence over American politics as a former presidential candidate, he'll have to think in very specific terms.

One obvious model for Sanders is Elizabeth Warren. Despite being in her first term, Warren is one of the most high-profile members of the Senate, with an ability to garner media coverage and shape the debate on the issues she cares about. Critical to Warren's success is her area of focus and expertise: financial policy, particularly as it relates to the way large institutions treat ordinary people. As someone who spent years as an academic studying issues like bankruptcy law, Warren has a deep knowledge of the subject and plenty of specific policy proposals (for example, the Consumer Financial Protection Bureau was her idea).

Can Sanders be that kind of senator, one who combines wonkishness with media savvy? It's unclear, but when he isn't running for president it will be harder for him to get attention. One thing he does have is a list of millions of supporters, who can, at least in theory, be called upon to exert pressure on Congress or the administration. And he can certainly work hard to help other progressive Democrats get elected. But all those people who gave him money or attended his rallies did so in the context of a presidential campaign, which is a unique kind of effort — one with a clear, shared goal and a finite duration. Candidates like Howard Dean and Barack Obama have tried to turn their campaigns into grassroots organizing machines with the ability to influence policy, and were met with limited success. The central challenge is that most of the voters who get worked up for a presidential campaign won't be as excited about a legislative debate or an argument about a presidential nominee.

And in his long political career, Sanders has always been more of a rabble-rouser than an organizer, which is fine, but suggests he'll be more likely to exercise influence as a single (if influential) voice and a presence in the media than as the leader of an organized body of activists.

So, let's imagine it's 2017. Sanders is back in the Senate, representing Vermont's 625,000 residents, as well as the dashed hopes of progressives from all over America. He'd like to get President Hillary Clinton to do the big things he advocated for in his campaign — break up the banks, institute single-payer health care, bring free college tuition to all. But Clinton has already rejected those ideas, for various reasons, and Sanders isn't going to change her mind when she's president. So he'll need other, more modest goals — specific ones that he can put into legislation or convince Clinton's administration to take up as their own, through whatever combination of persuasion and public pressure he can create.

Or he could wind up being the guy the TV networks call when they want to find a Democrat who'll criticize Clinton. There's no shame in that, but it's not the same as leading a lasting movement that produces change. If Sanders wants to the do the latter, he'll have his work cut out for him.