Bernie Sanders is on a roll.

He has won 6 of the last 7 Democratic nominating contests, including an impressive victory over Hillary Clinton in the Wisconsin primary on Tuesday. He outraised Clinton by $15 million in March and trails her by only one percentage point in the most recent national poll of Democratic primary voters.

As the campaign heads into the home stretch, Sanders has more momentum than ever.

But despite his recent victories, he still faces one daunting and inescapable problem: the Democratic superdelegates overwhelmingly support Clinton. Sanders simply cannot win without them. Even if he keeps his winning streak going, he has no chance of securing the nomination without persuading the superdelegates to switch sides.

Sanders’ superdelegate problem

“Superdelegates” are Democratic party officials and officeholders who will serve as delegates at the Democratic presidential convention. Under Democratic party rules, superdelegates may support whomever they want at the convention. Altogether they constitute about 15 percent of the total delegates. The remaining 85 percent are made up of “pledged” delegates, who are determined by the results of the primaries and caucuses.

Despite his impressive winning streak in the most recent primaries and caucuses, Sanders is trailing badly among both pledged delegates and superdelegates. Even after her Wisconsin defeat, Clinton still has about a 245-delegate lead among pledged delegates. Clinton’s lead is a direct result of the fact that she has won 2.4 million more votes than Sanders since the campaign began.

Sanders’ deficit among pledged delegates is compounded by the fact that Clinton also has a huge lead among superdelegates. She has, so far, the support of 469 superdelegates. In contrast, only 31 back Sanders. The upshot is Clinton has a commanding overall lead of about 685 delegates and is now rapidly closing on the 2,383 delegates she needs to clinch the nomination.

Accordingly, even if Sanders won every primary and caucus for the rest of the campaign, he would still lose the nomination to Clinton unless he manages to win crushing landslide victories in each of the remaining states. The reason is because under Democratic party rules pledged delegates are awarded on a proportional basis. For example, if a candidate wins the popular vote by a margin of 55 to 45, the losing candidate still receives 45 percent of the state’s delegates.

The proportional system thus makes it virtually impossible for Sanders to win the nomination on the basis of pledged delegates alone. Even when Clinton loses primaries, she wins delegates and gets ever closer to the magic number of 2,383.

That’s why the superdelegates are so important. If Sanders persuaded the superdelegates to defect from Clinton, he could erase her lead among pledged delegates. Not surprisingly, therefore, the Sanders campaign has begun a frantic effort to persuade the superdelegates to abandon Clinton.

So far, however, it isn’t working. Why not?

Why the superdelegates prefer Clinton

In theory, Sanders should attract strong support from the superdelegates. After all, many are just as liberal as he is. Moreover, there is historical precedent for superdelegates switching sides. In 2008, the superdelegates abandoned Clinton for Barack Obama, a memory that haunts the Clinton campaign.

But in the 2016 race the superdelegates and the party leadership have remained steadfastly committed to Clinton for three principal reasons.

First, many senior Democrats fear that Republicans would successfully typecast Sanders in the general election as an out-of-touch, weak on defense, left-wing extremist.

To be sure, Sanders’ bold brand of liberalism has resonated with young voters, a clear sign that the party is moving to the left. But there is no evidence that the nation as a whole is doing the same, at least not yet. A January 2016 Gallup Poll found that only 24 percent of Americans identify as liberal.

Moreover, although Sanders fares well in projected general election matchups against the Republican candidates, Democratic party professionals know that prominent liberal economists are on record as saying that his economic proposals make no sense. Those liberal critiques of Sanders would make for powerful and damaging GOP campaign television ads in the fall election.

So too will Sanders’ surprisingly weak grasp of the issues during an interview with the New York Daily News editorial board on April 1. In the interview, Sanders struggled with policy details and relied largely on talking points. The interview only compounded the perception among senior Democrats that Sanders is not a legislative leader. As the retired Massachusetts congressman and liberal Democrat Barney Frank recently warned

Bernie Sanders has been in Congress for 25 years with little to show for it in terms of his accomplishments.

Consequently, while Sanders’ platform of Democratic socialism and trade protectionism appeals to the party’s youngest and most liberal voters, older Democrats remember the 1968 to 1988 period when Republicans won five out of six presidential races. The memory of devastating defeats for liberal candidates haunts the Democratic party’s leadership and strongly inclines them to side with Clinton, a candidate widely viewed as a moderate like her husband.

Second, Sanders’ failure to win support among minority voters alarms the party’s leadership. Democrats lost badly in the last two midterm elections – 2010 and 2014 – because they failed to get minority voters and other key constituencies to the polls in sufficient numbers.

The majority of Sanders’s victories have come in small states like New Hampshire or caucus states like Minnesota where the Democratic electorate was heavily white, young, and more liberal than the nation as a whole. In contrast, Clinton has consistently won most of the big and diverse primary states, such as Florida, Illinois, Ohio, and Texas. The reason is because Clinton is winning the support of minority voters by massive margins over Sanders.

The idea of going into the general election with a nominee like Sanders who shows little appeal to minority voters is terrifying to Democratic officeholders. Without strong support and high turnout among African American and Latino voters, Democrats won’t win in November no matter who they nominate.

Finally, Sanders’ lack of significant foreign policy experience dismays the party leadership.

Thus far Sanders has benefited enormously from the fact that the Democratic primary campaign has focused almost exclusively on domestic issues. But Democratic officeholders know that in a general election, Sanders will not be able to escape addressing complicated and difficult issues like terrorism, Vladimir Putin, and the Middle East.

The few times Sanders has addressed global affairs he has not shown broad knowledge of the issues or strong grasp of policy details. As the New York Times recently observed, on foreign policy issues Sanders is out of his “comfort zone.”The fact is not lost on the Democratic leadership that voters view Clinton as far stronger on national security issues than Sanders.

New York is crucial

So is there any hope for Sanders?

The answer is yes, if he can defeat Clinton by a big margin in her home state of New York on April 19.

The intense media coverage of the New York primary will give Sanders a unique opportunity to articulate the specific details of his domestic and foreign policies. Equally important, a victory in a state as diverse as New York would prove that he has appeal to minority voters. Most important of all, a Sanders’ victory in Clinton’s home state would undermine the notion that she is a stronger general election candidate.

If Sanders can achieve all of those goals in New York, he has a chance to start winning over the superdelegates who are so critical to his campaign. But it won’t happen if he loses on April 19.

The Sanders campaign will rise or fall in New York.

Anthony J. Gaughan, Associate Professor of Law, Drake University

This article first appeared on The Conversation.