U.S. Democratic presidential candiate Sanders arrives to watch U.S. President Obama's State of the Union address to a joint session of Congress in Washington. Thomson Reuters

The sobering reality of Barack Obama's presidency has been the difficulty of achieving the change that he promised. The surprising development of the 2016 campaign is the degree to which a large segment of Democratic voters, at least in the early voting states, appear to have forgotten or rejected that lesson.

They seem willing to entrust their hopes of retaining the presidency to a candidate envisioning change far more radical than anything Obama ever dangled before them.

Bernie Sanders's supporters remind me of women who, once the baby is delivered, instantly forget the pain of childbirth and are prepared to do it all over again. Except that this analogy fails when it comes to the question of ultimate payoff. Why would voters, after watching Obama's excruciating experience with congressional Republicans, believe that Sanders could deliver his promised "political revolution"?

For all the fevered Obama-is-a-socialist rhetoric of Republican imaginings, the fact remains that he ran — and has governed — largely as a rather centrist, pragmatist Democrat. Sanders is an actual socialist.

Indeed, the 2008 primary campaign featured Obama running to the right of Hillary Clinton when it came to health care, one of the main differences between them. Back then, Clinton was the candidate who insisted that an individual mandate to obtain insurance was key to making an expansion of coverage feasible. Obama's campaign attacked her for a scheme to "go after people's wages."

Recall the hurdles, legal and political, that Obama and fellow Democrats faced in enacting Obamacare. Then ask yourself: How would Sanders manage the massively more daunting feat of enacting a single-payer health-care program? Or providing free public college and university tuition for all? Or raising the taxes necessary to pay for everything?

President Barack Obama watches then-Sen. Hillary Clinton give her Democratic National Convention speech in 2008. REUTERS/Jim Young

Of course, Sanders's answer to this challenge is to argue that Obama encountered stumbling blocks precisely because his approach was not revolutionary enough .

Obama's problem, in the Sanders analysis, was that he accepted the sail-trimming necessary to work, or try to work, within the existing system. Sanders envisions disrupting it.

More to the point, Clinton's campaign is premised on Obama's approach. She is all about the art of the possible, not the prospect of revolution.

Sanders's strategy is "not just that I have, gee, this idea for childhood poverty," Sanders recently told a Bloomberg Politics breakfast briefing here, dismissing Clinton's earnest incrementalism. Rather, it is to energize bottom-up change, with voters rising up against a corrupt and entrenched media-corporate-political establishment.

"What I will do — and it's not easy, I'm not here to tell you I have a magic formula — is start to make the United States Congress listen to what the American people want by mobilizing the American people," Sanders said.

"If voting turnout in America was 70, 75 percent, this country would be a very, very different place. Trust me. Because people, working people, young people, low-income people would be participating. Congress would have to listen to them, not just billionaire campaign contributors."

Good luck with that. In the run-up to the 2012 election, frustrated Obama allies diagnosed his difficulties as having come from spending too much time on an "inside Washington" game. He would, they vowed, take his case directly to voters, harnessing their energy to break the cycle of congressional intransigence. How'd that one work out?

Democratic Debate. REUTERS/Randall Hill

Even more, Sanders's confidence that the American people are overwhelmingly on board with his program — that the only thing blocking its enactment are those billionaire donors — is misplaced. Third Way, a centrist Democratic group, shared findings of a new poll of registered voters that raised questions about the power of Sanders's the-system-is-rigged message — even among Democrats, and certainly in a general election.

Given the choice of a candidate who promotes "helping Americans get ahead with more skills, more jobs and more wealth," or one who emphasizes that "the deck is stacked against everyday Americans and we need to focus on breaking up Wall Street banks and raising taxes on the wealthy," voters chose the growth message over the deck-stacked argument, 66 percent to 21 percent.

Just half favored breaking up Wall Street banks or raising the federal minimum wage to $15 an hour. Fewer than a quarter said they preferred "larger government providing more services," compared with 4 in 10 who wanted smaller government with fewer services.

Sanders says it will take a revolution. Americans are angry and frustrated, but it's far from clear they're prepared to embark on one.