MINNEAPOLIS – The Bernie Sanders phenomenon is at a turning point.

After blowing out all expectations, including his own, by moving to a decisive second place in the polls for the Democratic nomination and attracting huge crowds, Sanders now needs to convince Democrats he can be more than a protest candidate and actually win. He knows it.

“We need a movement which takes on the economic and political establishment – not one which is part of it.” Sen. Bernie Sanders

“Look, at the end of the day, people will say to me, we like you, we agree with a lot of your ideas, but can you win?” he told msnbc during a press conference Friday. “The issue of electability – can we win this election? Can we defeat the Republicans?”

On Friday, Sanders began to tackle that question in earnest with a speech to Democratic officials here that turned the issue of electability on its head.

“Let me be very clear,” he told the Democratic National Committee’s summer meeting. “In my view, Democrats will not retain the White House, will not regain the Senate, will not gain the House and will not be successful in dozens of governor’s races unless we run a campaign which generates excitement and momentum and which produces a huge voter turnout.”

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As evidence, he pointed to the Democratic drubbing in the 2014 midterm election, which he chalked up to the party running a “politics as usual” campaign that failed to excite minorities and young people.

“The Republicans did not win the mid-term election in November. The Democrats lost that election because voter turnout was abysmally low,” he said.

Not only did he promise that he could succeed in boosting turnout, but Sanders said he was the only candidate who could.

He pointed to the huge crowds that have greeted him on campaign stops across the country and support he has received online and in fundraising. A whopping 400,000 people have donated to his campaign, he said, far more than to any other candidate’s effort. And his campaign is only a few months old, he noted, suggesting that while many voters still don’t know who he is, his lack of name recognition also gives him plenty of room to grow.

To excite voters and get them to the polls, an establishment campaign won’t do, he said. “In other words, we need a movement which takes on the economic and political establishment – not one which is part of it,” he said, evidently referring to frontrunner Hillary Clinton.

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The message resonated with Sanders’ supporters, who packed the back rows of the ballroom and whooped during numerous standing ovations. But it may be a harder sell with the Democratic National Committee members.

After all, Democrats need look no further than the front row of the ballroom where Sanders was speaking to see a reminder of the danger of picking a strident liberal as their nominee: There sat Minnesota-native Walter Mondale, who received a standing ovation from the Democratic officials , but whose disastrous 1984 presidential campaign is often invoked as a warning.

Ronald Reagan crushed Mondale in the blowout election, winning every state and territory in the country except for the District of Columbia and Mondale’s Minnesota – and even there, the margin was just a few thousand votes.

“I think that it’s fair to say that few took our campaign seriously. But a lot has changed in these last few months.” Sen. Bernie Sanders

In 1992, Bill Clinton won by running a campaign that rejected Mondale-style liberalism and called for a more pragmatic and electable progressivism.

And while experts generally agree that Democrats lost the 2014 midterm because they failed to turn out young people and minorities, presidential elections naturally attract an entirely different electorate that tends to be younger, more diverse, and more Democratic.

“I think that it’s fair to say that few took our campaign seriously. But a lot has changed in these last few months,” Sanders said.

Without mentioning Clinton by name, Sanders suggested that she would not be able to turn out as many voters as he could.

“If there is a large voter turn out, Democrats will do well. If there is a small voter turnout, Republicans do well,” he told reporters after his speech. “I think you’re looking at the candidate who can substantially increase voter turnout all across the country.”