At Thursday’s Democratic town hall, Hillary Clinton unleashed a new line of attack against Bernie Sanders, saying, “Senator Sanders has also attacked President Obama. He’s called him weak; he’s called him disappointing. He tried to get somebody to run against him in the 2012 election in the primary… Maybe it’s that Senator Sanders wasn’t really a Democrat until he decided to run for president.”

Everything she said is true. In fact, she could go further. In 2006, after winning a Democratic nomination for Senate as a write-in candidate, Sanders declined to accept and said, “It would be hypocritical of me to run as a Democrat because of the things I have said about the party.” Sanders also once asked, “Why should we work within the Democratic Party if we don’t agree with anything the Democratic Party says?”

Yet here he is, after saying all that and more, coming within very real striking distance of winning the Democratic nomination for president. Which begs the questions, at least for establishment Democrats like Clinton: why did Sanders run as a Democrat and should longtime Democrats trust him?

There is an irony – granted, perhaps even a hypocritical one – to Sanders’s run as a Democrat. But the reason he was forced to do so actually furthers Sanders’s indictment of the political establishment and speaks to the central issue of his campaign, which is money in politics. In order to be taken seriously, he had no choice.

Third-party candidates don’t stand a chance in US presidential elections. In the last 30 years Ross Perot and Ralph Nader have made compelling runs, but no real dent. Perot won more than 18 percent of the popular vote in 1992 but didn’t carry a single state. Nader won about 2.74 percent of the popular vote in 2000 and his standing among the left still hasn’t recovered, as many blame him for costing Al Gore the election.

Running as a Democrat enables Sanders to access Democratic Party resources and appear on televised debates. It significantly raises his profile and keeps him from having to run as a third party who may be seen as a Democratic spoiler. Much as Sanders likes to rail that the system is rigged, he’s got to play by its rules to some extent if he wants his candidacy to be anything other than symbolic.

At this point it’s clear that not only is Sanders’s campaign far more than symbolic, the Democratic base has been eager for a candidate like him for a long time. Perhaps this is why many in the audience booed Clinton for her remark.

The Democratic Party does not get to give their voters the line; voters must give it to them. No thinking person should be told their position by a political party. Plenty of liberals have been disappointed with President Obama; supporting everything he’s done should not be a requisite for the Democratic nomination. What enriches democracy is the vibrancy and variance of opinions pooled together and pitted against one another until the best one wins.

Nothing on Sanders’s agenda is incompatible with what ought to be the Democratic platform – supposing the Democrats are still the party of the left. In fact, Sanders’s run as a Democrat, even if unsuccessful, is undeniably a good thing for the party. For too long the party has been hostile to true progressives. Establishment Democrats really don’t like anyone to their left and the party has held to a pretty firm limit of how far it’s willing to go, bullied by Republicans and bought off by big business.

By taking this line of attack on Sanders, Clinton is essentially saying what everyone in America is so sick of hearing: party affiliation matters and the establishment knows best how to run things. Not only is Sanders correct in his critical assessments of the Democratic Party, many within the party agree and have felt disenfranchised by the party for years. Clinton is not entitled to insinuate that Sanders, who now has nearly as much support as her among party voters, doesn’t belong.

For Sanders supporters there may be fear that running within the system will somehow corrupt their candidate. Perhaps as president Sanders will compromise some of his campaign promises. But a compromise from Sanders’s progressive position is a lot better than a compromise from Clinton’s centrist one. Even if he doesn’t win the nomination, forcing the Democratic Party leftward is time well spent, whether establishment Democrats approve or not.