Bill Scher is the senior writer at the Campaign for America’s Future, and co-host of the Bloggingheads.tv show “The DMZ” along with the Daily Caller’s Matt Lewis.

While Sen. Bernie Sanders has taken the first step in any successful liberal insurgency, striking a nerve with a faction of voters, he appears to be on the verge of bumping up against his natural ceiling. To go from having his 15 minutes to being a true contender, the Vermonter is going to have to get out of his comfort zone. Maybe lay off the single-payer health care, drop the trillion-dollar price tag on his jobs bill or even *gasp* start a Super PAC.

Though Sanders is within 10 points of Hillary Clinton in New Hampshire and gaining quick in Iowa, a recent NBC/Wall Street Journal poll suggests limited room for growth. When asked “could you see yourself supporting” different candidates for president, only 40 percent of Democrats would even entertain the possibility of supporting Sanders. By contrast, a whopping 92 percent can imagine backing Clinton.


What’s holding him back? The answer is simple: electability. In a recent Bloomberg poll, which pegged Sanders’ support at 24 percent in Iowa and New Hampshire, only 12 percent of Democrats in those states expressed confidence that Sanders could beat the Republican nominee. That means half of his own supporters don’t believe he can win the White House.

Proving general election viability is an essential step all liberal insurgents have to take to build support. The proof typically comes in the form of ideological restraint, signaling that candidate knows how to bend when necessary. That’s not Bernie’s strong suit, but he might want to consider how the last couple of Democratic upstarts managed to usurp the frontrunner mantle.

In the 2008 campaign, Barack Obama had to leaven the “audacity” of his pledge to conduct direct diplomacy with Iran and North Korea, with disavowals of past positions on single-payer health care, Palestine and gay marriage. In the spring of 2003, Howard Dean’s pollsters learned that, while his opposition to the Iraq War got him off the ground, he needed to talk up his more moderate gubernatorial record of balancing budgets if he wanted to attract new supporters. He did, and became the frontrunner for much of the race, despite flaming out in the Iowa caucuses.

Only George McGovern in 1972 managed to crash the party without trimming any of his ideological sails. But he is an exceptional case; he had the unusual advantage of having literally rewritten the party rules for selecting delegates to the Democratic national convention, rules that his opponents failed to fully grasp. (McGovern actually won fewer primary votes than his opponent Hubert Humphrey, yet won the most delegates.)

The other method insurgents use to convince voters of their electability is raising tons of cash. Democrats may hate money in politics, but they hate losing more. And their never-ending paranoia of being swamped by wealthy Republican donors overrides all other considerations.

Dean was the first to prove that Democratic voters were more impressed by big dollar tallies than by nobly staying within the public financing system’s voluntary spending caps. When Obama followed suit, the collective sighs of relief among Democrats drowned out the few voices carping about the selling off of our democracy, not to mention the accusations from Sen. John McCain that he broke his word.

Even McGovern had his sugar daddies. While he was a pioneer in small-dollar direct-mail fundraising, the New York Times reported that he also received “millions” from a “small group of young millionaires,” including the heirs to the Max Factor and General Motors fortunes, and Wall Street titan John Gutfreund. McGovern needed every penny too. In the pivotal California primary where he effectively ended Hubert Humphrey’s campaign, McGovern outspent his rival by more than 8-to-1 in order to win by 5 points.

Yet for Sanders to either restrain himself ideologically or fully engage the money race in the name of political expediency cuts against the fundamental premise of his candidacy. He is running to legitimize his vision to “redistribute [the] wealth to the middle class.” He is running to redefine what policies are considered politically acceptable. And he is running to galvanize grassroots Americans to rid the political system of Big Money, refusing to set up a Super PAC or solicit major donations from the millionaires who do support him.

In other words, he is trying to defy the laws of political gravity.

Of course, Sanders and his most loyal fans may be convinced that this time is different. Populism is ascendant. Conservatism is discredited. Disgust with rigged politics is peaking. And the growing gap between the one percent and everyone else has opened up the political space for bigger and bolder solutions.

All true. As I recently wrote, the center of gravity in today’s Democratic Party, and perhaps the entire country, is moving leftward. That makes it tempting for Sanders to emulate McGovern and hope to shoot the ideological moon.

But it is dangerous for confidence to bleed into hubris. An electorate demanding more government action may be primed to accept proposals that would eliminate student debt, make preschool fully accessible and guarantee paid leave, yet could still recoil at single-payer health care, a top tax rate over 50 percent and a $1 trillion price tag for a transportation infrastructure bill.

Those are all proposals Bernie wants to pursue. But each comes with major warning signs. Single-payer couldn’t get off the ground in the one state that tried: Sanders’ Vermont. A 2012 poll found that only two percent of Americans supports a top tax rate above 45 percent. And one of the few Democratic bright spots in the 2014 midterms, Michigan Senate winner Gary Peters, won with an anti-spending message: “you don’t spend money you don’t have.”

A successful insurgency punctures the false limitations of conventional political wisdom: Obama’s direct diplomacy gambit was thought to be a gaffe by pundits but proved to be a useful wedge to distinguish himself from Hillary Clinton and John McCain. Still, Obama picked his spots, and cut loose any dead ideological weight from the past that might hinder his path, knowing he could always resurrect some of those old views when the timing was right.

Sanders is not above such Machiavellian maneuvering. As is now being recounted, he has occasionally danced with the NRA to curry favor with his rural hunting constituents. He ably compromised to push through legislation creating audits of the Federal Reserve (after which Ron Paul said he “sold out”) and he gave ground when reforming the Veterans Administration health care system. (Sanders could play up his pragmatic side and argue that’s two more pieces of major bipartisan legislation he passed than Hillary Clinton ever did.)

But Sanders would have difficulty retreating on any of the economic issues that are most dear to his heart, issues where he has staked out positions that would be essentially impossible to defend in a general election. Perhaps a successful insurgent could get away with pushing the envelope and marshaling political capital around one or two big controversial ideas. But trying to carry the entire progressive wish list is too much for any presidential candidate.

At least, it would be for any presidential candidate trying to win, which may not Sanders’ end goal. He may recognize that he doesn’t stand enough of a chance against Hillary to warrant the reputational sacrifice of watering down his agenda to appease the majority of Democratic voters who don’t describe themselves as “economically liberal.”

There was a common thread in the McGovern, Dean and Obama insurgencies—an active war deeply unpopular among Democratic voters yet abetted by their Democratic opponents. Sanders has no such opening against Clinton, whose Iraq War vote is now a distant memory. The fact that nine out of ten Democrats can see themselves backing Clinton indicates a lack of a significant “Anyone But Hillary” constituency.

If Bernie privately views defeating Hillary as unrealistic, then it makes more sense from him to remain unwavering in his platform. If the Democratic Party is ever going to be able to run on ideas that cost real money and require higher taxes, someone has to take the lead and make the case. Someone needs to put proposals on the table that make people uncomfortable today so that those ideas will feel comfortably normal tomorrow.

If a long-term shift in the discourse is what the Senator hopes to accomplish, then there is no need for him to change what he is doing. But if he means what he says when he predicts victory in Iowa, New Hampshire and beyond, then he’ll need to stop preaching like an activist and start campaigning like a winner.