In 2008, then-Senator Barack Obama promised to unite Washington and the nation behind progressive change. Then-Sen. Hillary Clinton mocked him.

"The skies will open, the light will come down, celestial choirs will be singing and everyone will know we should do the right thing and the world will be perfect," she said. "I have no illusions about how hard this is going to be."

Now, as Clinton runs for president again, many of her allies believe her cynicism was vindicated and are feeling deja vu.

The former secretary of state is trying to convince voters once more that her opponent -- this time, Sen. Bernie Sanders (I-Vt.) -- is promising a unicorns-and-rainbows vision of governance. She's hopeful that seven years of partisan warfare will help her make the case. And she has Capitol Hill veterans and even some Obama veterans echoing her larger points.

"I think Bernie is giving voice to legitimate and passionate concerns that people have about injustices in our economy and society," said David Axelrod, a longtime Obama adviser. "But when it comes down to the nitty gritty of governance, I think even he knows -- my impression speaking with him -- the limitations of governance as well."

With just days to go before Iowans caucus, the Democratic primary has morphed from a debate over progressive agendas into an argument over how to get those agendas into law. Sanders says his formula is simple: He won't just win the presidency, he will shepherd in a "wave" of Democrats who will provide him momentum to change the country.

"If he does win it is going to be such a shock to the system that we will not stop the fight the day he wins, we will pursue it vigorously," Sanders adviser Tad Devine told The Huffington Post. If Congress proves resistant to Sanders' appeal, "we will turn the midterm elections in 2018 into the largest referendum in the history of midterms," Devine added. "We will take [Republicans] on frontally, and trust me, [Bernie] will be up for the fight."

But as he wins over voters with talks of a "revolution," the senator is leaving a group behind. Many Democrats in Congress and in the administration aren't persuaded by the Bernie Sanders theory of change.

"The Bernie people believe you need a stubborn, idealistic fighter for big progressive ideas who will win the fight to keep the base motivated and energized," said one longtime Capitol Hill aide. "If they're right, they hold the key to doing what neither Barack Obama nor Bill Clinton ever did."

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Obama, after all, had also promised to galvanize popular support for his agenda. After the 2008 election, his campaign structure morphed into Organizing for America, an entity meant to coax, convince or intimidate lawmakers into supporting the president's legislative pursuits through district-based lobbying campaigns.

It was great in theory. Not so much in practice. "Not at all," said one Democratic lawmaker when asked about OFA's effectiveness on the Hill.

One reason Obama was unable to more effectively utilize an outside game was that "there was a lot of work," as he told Politico's Glenn Thrush this week. "I didn't oftentimes have the luxury of a six-month run-up and then a two-month or three-month victory lap because, you know, 'You saved the auto industry? All right. What's the next thing?'"

But on the big-ticket items, the president also strategically chose to get Congress' buy-in and not its submission.

Health care reform was crafted transactionally -- a deal with the pharmaceutical industry, trade-offs with lawmakers -- and it worked. The Affordable Care Act became law after decades of failed reform attempts. Climate change legislation met a worse fate. In 2010, midwestern Senate Democrats declined to move on cap-and-trade legislation after it passed the Democratic-run House. Obama dropped the matter -- the prospect of an electoral bloodbath (which came anyway) undoubtedly influencing his thinking.

We will turn the midterm elections in 2018 into the largest referendum in the history of midterms. Sanders' top adviser Tad Devine, on what will happen if Republicans stand in his way during the presidency.

Sanders' theory is that a mass movement behind these bills could have led to better results. And he, as well as many other progressives, has long argued that Obama suffered midterm losses not because he pursued those items but because he didn't do so with enough vigor.

"Assuming that the White House is good and frames things right, you can get people really excited and make the Republicans look like absolute dog shit," said Mike Lux, a longtime progressive operative who served Obama during his presidential transition. "I think what happened the first time around is the Obama folks decided on an inside strategy ... and it deadened everybody. I've joked with them that they’re the only people in American history who could have passed a health reform bill and have the progressive base mad at them. It is almost an impossible feat but they pulled it off."

The problem, as others in the party see it, is that politics often isn't a binary choice between Republicans and Democrats. The Affordable Care Act wasn't held up just because of conservative opposition. It was moderate Democrats who demanded it be watered down (goodbye, public option) and complained over items in the bill that affected their local interests (the medical device tax).

"I think as a matter of general philosophy, outside pressure is quite important. But the truth is, legislators are a lot more interested in hearing from people who can vote for them than they are from hearing from their colleagues," said Delaware Gov. Jack Markell (D). "Obama was the ultimate change candidate. And he had OFA behind him, which I think was not insignificant. But this just shows the real world, which is that there are a whole lot of districts out there. Not everybody is representing San Francisco."

Even if a President Sanders could keep Democrats in line, he'd have many hurdles to clear before scoring legislative wins. Sixty votes are needed to get most things done in the Senate. And redistricting has not just given Republicans firm control of the House until 2022, it's made the party's members more responsive to their ideological base.

"The pressure that you could apply was not going to move Republicans," Axelrod said. "In a sense Obama was both the beneficiary and victim of his own success. By the time he got to Congress, a lot of moderate or swing Republicans were eliminated. And so, that caucus was implacably opposed."

Having been in Congress for nearly 25 years, Sanders is keenly aware of the institution's gridlock. And he's managed to operate successfully around it on occasion, scoring one of the biggest bipartisan achievements of Obama's second term -- a bill that, ironically, moved veterans' care further from the government-centric system that Sanders wants to apply to health care nationally.

A Sanders presidency, as Devine noted, wouldn't be strict political combat. "He can switch-hit on this," but Republicans would "have to meet him halfway" first.

But here, too, there is skepticism -- not over whether Sanders would make that turn, but whether he could after spending a career and a campaign promising bold, uncompromising pursuits.

"You can see by the fact that he has no endorsements from his colleagues, governors too, Sen. Sanders would have to play entirely an outside game," said former Sen. Kent Conrad (D-N.D.), who endorsed Obama over Clinton in the 2008 primaries. "And to me that is not as effective as doing both. You have to build public support for positions. But you also have to have relationships. Sen. Sanders, because he is a self-described socialist, has put himself out on the edge of American political positioning. People will be, 'Oh you compromised with a socialist? Or you're supporting a socialist?'"

Carlos Barria / Reuters

In recent weeks, frustration with Sanders' revolutionary zest has boiled over at Clinton headquarters. "I wish that we could elect a Democratic president who could wave a magic wand and say, ‘We shall do this, and we shall do that,’" Clinton declared recently, in a comment that echoed the celestial choirs one from eight years prior. "That ain’t the real world we’re living in!”

And her campaign has begun more forcefully arguing the case that Sanders' vision is fanciful.

"Sen. Sanders has a habit of being unable to answer the follow-up question about how he is going to get any of this done short of invoking a so-called political revolution," said her spokesman Brian Fallon. "When you are campaigning for president you owe the voters more than just a platform that represents your idealized set of circumstances. You owe them an explanation for how you are actually going to achieve results to make a difference."

Senators Sanders has a habit of being unable to answer the follow-up question about how he is going to get any of this done short of invoking a so-called political revolution. Clinton spokesman Brian Fallon

For the Sanders campaign, the explanation really is simple and not at all undercut by the last seven years. Obama's experience doesn't vindicate Clinton's cynicism, Devine argues. Instead, it shows a president who had to abandon the outside game out of circumstance and who -- admirably, though perhaps unwisely -- proved too accommodating to the opposition.

"Bernie is going to be left with a much better situation than the one Obama inherited. There is not a crisis where the economy may collapse any day," said Devine. "He is also very much aware of the Republican strategy of obstructionism. He understands it completely and he is willing to engage on it in the electoral battleground at the congressional district level."

This logic has clearly proven persuasive to voters. Sanders now stands a reasonable shot of winning both Iowa and New Hampshire, and Democratic Party insiders are talking more openly about the prospects of a months-long nominating process.

All of which, they insist, is fine (certainly, it didn't hurt Obama in 2008). The worry is about what comes afterward.

"Bernie is terrific. The thing is, he is pure in where he is and you can always deal with someone pure in their philosophy," said Penny Lee, a longtime adviser to Senate Minority Leader Harry Reid (D-Nev.). "The fault of Bernie is that there is no setting of expectations. He has an ideal and he wants to see America operate in the ideal of the issues he is fighting for. ... But it would be a struggle, because you have an intransigent other party that has a polar opposite view of where he is on many issues. You’d be setting his voters or supporters up for some disappointment."