SIOUX CITY, Iowa (AP) — Bernie Sanders was halfway through his opening statement — a stern, 45-minute lecture on domestic policy leavened with a dash of political pep talk — when he realized the crowd had missed one of his rare attempts at humor.

"That was a joke!" he bellowed. Laughter briefly rippled through the audience as the Vermont senator returned to his statistic-rich pitch for increasing the federal minimum wage to $15 per hour and breaking up big Wall Street banks.

Sanders' down-to-business demeanor on the campaign trail belies the youthful enthusiasm that's accompanied his unexpected rise in the Democratic race for president. With less than two weeks until voting begins, the 74-year-old socialist could win both Iowa and New Hampshire, a once unthinkable outcome in a primary campaign that was supposed to be tailor-made for Hillary Clinton.

"Today the inevitable candidate doesn't look quite so inevitable," Sanders told voters who braved icy roads and single-digit temperatures to see him speak Tuesday morning in Fort Dodge.

While Sanders first garnered attention for the overflow crowds he drew around the country last summer, he's making more intimate appeals to voters in the final few days before Iowa's Feb. 1 caucuses. On Tuesday, he slipped into a luxury bus ("You can walk around, it's got comfortable seats," he said of his new ride) for a day of smaller town hall meetings across central and northwest Iowa.

As voters filed into the events, a campaign soundtrack played a heavy rotation of songs touting revolution, a nod to Sanders' call for a "political revolution" in America. Singer Tracy Chapman's "Talkin' Bout a Revolution" is a favorite, as is the band Flogging Molly's "Revolution."

Young people with nose rings, green and purple tinted hair, and an array of hoodies, filled the seats directly behind the candidate, though the rest of the audience skewed older. A campaign supporter in Carroll used a Robert Frost poem to introduce the senator, urging supporters to make sure Sanders' road "less traveled" takes him to the White House.

Yet there's little lightness once Sanders begins to speak — no amusing anecdote to open his remarks, save for a small quip about how Iowa's winter reminds him of his home state. Sanders leans into the lectern and his Brooklyn-accented voice quickly reaches a shout. He repeatedly jabs his index finger at the audience, and when he's ready to make a point of emphasis, he lifts both arms into the air as if conducting an orchestra.

No one would credit Sanders with the timing of a comic: His crack, about Wal-Mart being a beneficiary of the welfare state, came across as just another plank in his anti-corporate platform.

Sanders, an independent who caucuses with Democrats on Capitol Hill, began his campaign with firm rules about what he was not willing to do to win the presidency. He's among the most vigorous critics of super PACs, political groups that can accept donations of any size, and frequently boasts about his campaign's reliance on small donations. He also vowed to avoid personal attacks on his rivals.

But with the prospect of victory in the early states, Sanders is testing the limits of that latter pledge. In addition to his criticisms of Clinton's evaporating inevitability, he relishes pointing out the big-money speaking fees Clinton received from Goldman Sachs, the Wall Street giant that's a frequent villain in Sanders' speeches.

"Goldman Sachs also provides very, very generous speaking fees to some unnamed candidates," he said during a town hall meeting at a winery in Carroll, just across the street from Clinton's campaign office in town.

While Sanders may be striking a chord with voters seeking an outsider candidate, he's also a practiced politician who is acutely aware of his standing in the Democratic race.

Taking a page out of Republican front-runner Donald Trump's playbook, Sanders has recently started opening his remarks with lengthy references to his improving poll numbers. He's particularly fixated on surveys showing he's more likely to beat Trump in the general election than would Clinton, underscoring his irritation with suggestions from within his own party that's he's unelectable.

He also knows he lacks foreign policy experience, particularly when his record is stacked up next to Clinton's four years as secretary of state. So he reaches for an easy applause line with liberals, reminding voters of Clinton's vote in favor of the 2003 invasion of Iraq — then noting former Vice President Dick Cheney also had foreign policy experience.

Even if Sanders can turn his momentum into victories in Iowa and New Hampshire, he faces daunting challenges ahead. He's less well known among minority populations, which some Democrats see as a vulnerability for him as the race heads to states with more racially diverse populations.

Sanders seems well aware of this, too. He's making an effort in South Carolina in particular to reach out to black voters, hoping that like the crowds greeting him in snowy Iowa, they'll see the rumpled socialist as their advocate.

"This is it. Here I am," Sanders said as he closed an event in Iowa. "For better or worse."

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Follow Julie Pace on Twitter at: http://twitter.com/jpaceDC