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This article originally appeared on The Huffington Post.

The mainstream media continues to be shocked that Bernie Sanders keeps gaining traction against frontrunner Hillary Clinton. However, if you look at what Sanders actually stands for, it is well within the mainstream of what used to be the Democratic Party.

Ever since Jimmy Carter, it has been evident that much of the Democratic electorate, and for that matter much of the country, is more progressive in its core values than what Democratic presidents have been offering. As big money has crowded out grassroots democracy, the policies that people crave are simply not on offer.

There is also the historical accident of which leaders arise at what moments. We have not had a large number of plausible progressives with national appeal.

In 2008, John Edwards, who looked to be a pocketbook progressive, turned out to be a phony. A lot of people who voted for Barack Obama thought they were supporting a more progressive candidate than Hillary Clinton, but they turned out to be wrong.

Go back through the history of the Democratic Party primaries for the past half-century, and look at what happened to progressives. In 2004, Howard Dean rallied the popular hunger for someone "from the Democratic wing of the Democratic Party," but he turned out to be a flawed candidate.

Paul Wellstone? Died in a plane crash. Bobby Kennedy? Assassinated. Fred Harris and Mo Udall, both progressives who ran in 1976, crowded each other out. As politics has been captured by elites, we have not had a rendezvous of the right leader with the right moment.

Meanwhile, the American economy has turned viciously against ordinary people. Banks, corporations, and the 1 percent have more power than ever, political as well as economic.

So there is a pent-up demand for a candidate who can articulate popular frustrations.

The fact that a 74-year-old, self-described socialist transplant from Brooklyn to Burlington, Jewish no less, is the surging vessel of these demands only tells you how deeply felt they must be.

But Bernie is no more radical than, say, Harry Truman, FDR, or LBJ (when he was thinking about domestic policies). My friend Peter Dreier, a few months ago, performed a real service when he compared key Sanders positions with public opinion generally.

As Dreier reported, overwhelming majorities of Americans support a higher minimum wage: 74 percent think corporations have too much influence; 73 percent favor tougher regulation of Wall Street; 58 percent support breaking up big banks; 79 percent think the wealthy don't pay their fair share of taxes; 85 percent favor paid family leave; 80 percent of Democrats and half the public generally support single-payer Medicare for all; well over 70 percent of Americans support workers' rights to unionize; and on and on.

No wonder Sanders is gaining ground.

Republicans have been disparaging Democrats as socialists-even centrist ones like Barack Obama-ever since FDR. So if this be socialism, let's make the most of it.

Sanders and Clinton are both rock solid on civil rights and on LBGT rights. But it's on the pocketbook issues that Bernie out-flanks Hillary. The fact that he raises his money mainly from small donors while she gets it from the usual suspects underscores the difference.

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The press also likes to compare Bernie Sanders with Donald Trump. Superficially, both reflect mass frustrations with economic unease and political blockage. But Trump represents know-nothing celebrity politics, while Sanders is actually serious about ideas and policies. What they have in common is the threat that they pose to politics-as-usual and to the bipartisan elites.

It's not at all clear that Sanders can be nominated, much less elected. But he does create major problems for Hillary Clinton. The idea that Joe Biden could get into the race and save the day for what passes for the Democratic center is preposterous. Biden would dilute the Clinton vote, not the Sanders vote.

Some of us have argued that Elizabeth Warren could be a more electable version of Bernie Sanders, but it's hard to imagine a scenario that gets her into the race.

And the more Sanders keeps gaining appeal, the less likely it is that he would get out of the way for Warren.

Leaving aside the horse-race aspects, which are hard to resist, here's the point: Sanders represents the pent-up demand of rank-and-file Democrats to get their party back.

Half a century ago, such ideas as full employment, a strong labor movement, national health insurance, investments in early childhood, free higher education, ending poverty in the richest nation on earth, progressive taxation, large-scale public infrastructure outlay, effective consumer regulation, and full enforcement of civil rights were utterly mainstream. Guess what? They still are.