Hillary Clinton's quest for the Democratic presidential nomination in 2016 was supposed to be a coronation, not a conflagration.
While Mrs Clinton has spent the past year building a campaign infrastructure that she hopes will carry her to success in Iowa and beyond, Mr Sanders has become a fundraising powerhouse, thanks to passionate grass-roots support.
The Clinton camp may have had a head start on organizing support for Monday's caucuses, but the Sanders campaign is pouring its newfound resources into the state in hopes that a win here - and in New Hampshire next week - will vault him into a national lead and position him to prevail in a fight for the nomination that could drag on for months.
The pitched battle between the two candidates was on full display on Friday night, as they held rallies mere blocks from each other in Davenport, Iowa. Although they drew similarly sized crowds of about 1,000 people in this modest-sized city on the banks of the Mississippi River, it was a clear contrast in styles.
Outside an old-fashioned dance hall, Sanders loyalists lined the street, waving homemade signs, posing with a life-sized cutout of their candidate and cheering as passing cars honked their support.
"I'm so excited to change the world," said Selina Vickers, a social worker who drove 11 hours from Fayette County, West Virginia, to help with the get-out-the-vote effort. "I've never been inspired like this."
Mrs Clinton's backers - a relatively older crowd - were more restrained as they waited to enter their auditorium a few hours later. No car horns, no cheering - just a long line to get through a door guarded by Secret Service officers.
"I'm proud to be an American, where at least I know I'm free ..." the singer belted out.
Mr Sanders was introduced by Cornel West, the fiery author and black rights activist. He shouted, clapped and practically pirouetted, telling audience that the Vermont senator was at the centre of a political movement, a "moral and spiritual transformation" in the nation.
"We're trying to remake and recast American democracy, and Bernie Sanders is the select one," he said.
A hoarse former President Bill Clinton was his wife's warm-up act.
We need someone who is strong and smart, who can make things happen," he said. "You can talk about change, but somebody's got to make it."
Those two introductions illustrated the difference between the two candidates. One is a veteran political insider with a famous last name. The other is a hero of the progressive movement, who - as a self-professed "democratic socialist" - until last fall wasn't even a registered member of the Democratic Party.
On the stage in Davenport, Mr Sanders promised sweeping changes: that his election will herald a revolutionary development in US politics, when those who are struggling in the modern economy will wrest power away from what he derides as the "billionaire class". He called for free college education for all Americans, breaking up "too-big-to-fail" banks, a government-managed healthcare system and raising the minimum wage.
Critics, he said, insist "it's too hard, it can't be done, we have to think much, much smaller. And I say the hell with that!"
The day before, at an appearance before high school students in Des Moines, Mr Sanders' college proposal received an enthusiastic audience - more confirmation of polls that show the candidate doing much better than Mrs Clinton among younger voters.
Ms Clinton, on the other hand, preached practicality during her Davenport speech. Politics, the saying goes, is the art of the possible - and Mrs Clinton frames herself as the candidate who knows how to work within the system.
She said she would build on the healthcare reforms President Barack Obama has already put in place, use existing law to rein in Wall Street excesses, and make college more affordable without giving free tuition to the wealthy.
"We've got to be smart, and we've got to run an election about the real issues and what we can get done," she said. "I'd rather under-promise and over-deliver than the opposite."
The former secretary of state's speech came at the end of a long day of campaigning across Iowa, and if her pledge to "under-promise" and grind out results isn't the stuff of political revolutions, it was a theme cited again and again by her supporters.
"I like Bernie's ideas, but I really just think Hillary can take on the Republicans," Lindsey Cowherd said, at a rally at in a college gymnasium in Des Moines. "She has the qualifications from day one to do a great job."
Republican pollster and political speech expert Frank Luntz says, "I think Bernie Sanders wins the caucuses". "It's energy, passion, motivation. They want to change America, and they think they can change the world."
If Luntz has his suspicions that Mrs Clinton is in trouble - both here and in New Hampshire, which will hold its primaries a week after Iowa - that may be confirmed by increasingly sharp criticism Mr Sanders is taking, not just from Mrs Clinton and her supporters, but from other gatekeepers in the Democratic establishment.
On Saturday the New York Times said Mr Sanders' proposals "aren't realistic" and endorsed Mrs Clinton for the nomination, writing that she would be the "most broadly and deeply qualified presidential candidates in modern history".
On Thursday the Washington Post editorial board called Mr Sanders "a politician selling his own brand of fiction to a slice of the country that eagerly wants to buy it".
"Mr Sanders's success so far does not show that the country is ready for a political revolution," the paper wrote, citing the lack of details behind his healthcare, education and tax plans. "It merely proves that many progressives like being told everything they want to hear."
He said Mr Sanders has the benefit of being a fresh, new face for Iowa voters, which may explain the enthusiasm of his crowds, but Mrs Clinton has broad support across the state. While Hillary Clinton is an established political figure, less is known about Vermont senator Bernie Sanders. Here are five things to know about him:
He is a socialist. Sanders is running as a "Democratic socialist", but in his long political career he became comfortable with just "socialist".
Climate change is real. After the National Oceanic and Atmospheric Administration marked 2015 as the hottest year on record, Sanders tweeted: "The debate is over. Climate change is real and caused by human activity."
College should be free. "A college degree is the new high school diploma," Sanders wrote in a Washington Post opinion piece.
Gun ownership is a "lifestyle that should not be condemned". Sanders' record on gun control has been mixed which he says is due to the fact that his constituents in Vermont are pro-gun and "99 percent of the people in my state who hunt are law abiding people".
Mrs Clinton is hoping that after eight years of Mr Obama's presidency, where his earlier idealism often has been tempered by the reality of modern American politics, Democratic voters are willing to be practical.
Mr Sanders, on the other hand, argues that Democrats need to think much, much bigger.
It's a debate that won't be resolved when Iowans head to the caucuses on Monday. For Democrats, this is just getting started.
And in theTrump Camp?
Donald Trump leads Texas senator Ted Cruz by five points in the Republican presidential nominating race in Iowa, just two days before the state’s caucuses, according to a gold standard Des Moines Register–Bloomberg News poll published on Saturday evening.
On the Republican side, the poll had Trump at 28 points and Cruz at 23 points, with Florida senator Marco Rubio at 15 points and retired neurosurgeon Ben Carson at 10.
On the Democratic side, former secretary of state Hillary Clinton led Vermont senator Bernie Sanders, 45-42, according to the poll, which surveys about 400 likely caucus-goers.
The poll is known for picking up on Iowa election dynamics that outsiders miss. This time the dynamic is that Clinton remains strong and Trump has fought off a challenge from Cruz.
Clinton was six points ahead of Sanders in the same poll in December, but only two points ahead of him earlier this month.