The political revolution has arrived in New Hampshire, as Bernie Sanders easily defeated Hillary Clinton in Tuesday's Democratic primary, according to calls by multiple networks.

Sanders's victory — which would have been all but unimaginable a year ago — is a truly remarkable achievement for a "democratic socialist" who began the campaign as a mere blip in the polls, little-known nationally and lacking any party establishment support whatsoever.

The Vermont senator's triumph is a testament to the power of his economics-focused message, to his supporters' enthusiasm and organization, and to his wild popularity among young voters. It's also a stinging rejection of the Democratic establishment and Hillary Clinton by primary voters in the Granite State. And it's a strong follow-up to Sanders's tie with Clinton in last week's Iowa caucuses.

Some will argue that Sanders's win isn't that big a deal, since he's from the neighboring state of Vermont, and it's long been known that Sanders does well among the white Democrats overrepresented in New Hampshire. Yet it's worth remembering that when 2015 began, Sanders trailed Clinton by around 40 points in the Granite State. And though tonight's votes are still being counted, the early calls in Sanders favor suggest that it isn't even close.

Still, all this doesn't mean Sanders is favored to win the Democratic nomination. To pull that off, there's one big thing he still needs to do: hugely improve his numbers among the nonwhite voters who are so crucial to the party's coalition nationally. In the coming days, we'll see whether he can make that happen.

Sanders's campaign has been remarkably successful so far

Back in September 2014, I watched Sanders hold a town hall at Waterloo, Iowa, and candidly discuss his misgivings about the presidential campaign he was considering.

"I have to be realistic," he told the crowd. "I don't want to run and make a fool of myself, or, most importantly, do a disservice to all of the ideas that are needed. Believe me, if I ran with this program in a campaign and did very poorly, it would be a disservice to those ideas." He continued: "So if you do it, you've got to do it well and do it to win. That's tough stuff! That is very, very tough stuff."

By now it's safe to say that Sanders's worst fears weren't vindicated. Far from it — instead, his campaign has done wildly better than practically any political observer predicted, and is helping shape the future of the Democratic Party.

A longtime independent and "democratic socialist," Sanders is calling for the Democratic Party to move to the left on economic and domestic policy issues — embracing single-payer health care, funding college tuition for all Americans, and hiking government spending on infrastructure and Social Security benefits. And he's argued that because recent Democratic leaders have been too centrist and too reliant on fundraising from business interests, the American public has lost faith that the party will fight for them.

If his campaign's success is any indication, a lot of the party's voters find this critique really convincing. Sanders has proved popular enough to raise massive amounts of money from small donors, enough to let him go toe to toe with Clinton, who's backed by the entire Democratic establishment. And his supporters showed up to New Hampshire polls in droves on Tuesday.

Why Sanders won New Hampshire and tied in Iowa

Over the past few months, I've interviewed dozens of Sanders supporters in Iowa and New Hampshire. And when I asked them why they supported Sanders, several common themes came up.

First, there was a belief that something is very wrong with America, and that serious change is required to fix it. The underwhelming economy, growing wealth inequality, and the disproportionate power of corporations and the superrich came up again and again. "I'm supporting Bernie because of his commitment to changing the dynamic in Washington, dealing with this wealth inequality, and helping people at the bottom," said Fran Berman of Exeter, New Hampshire.

Secondly, they argued that Sanders's campaign and his call for a political revolution presented the possibility of major change. "The status quo will only make changes that benefit the people in power," Bob Moore of East Kingston, New Hampshire, told me. "We either do this or we become more oligarchic, so I don't think our democracy has any other choice," said Ron Yarnell of Johnston, Iowa.

And finally, many said they simply trusted Sanders more than Clinton and the Democratic establishment. "This guy believes in what he says. And he speaks from his heart, and you can believe in what he says," said David Lancaster of Hopkinton, New Hampshire. "All you can think of with Hillary is, she’s going, 'What are the polls and what are my political advisers telling me I should be saying?'"

What Sanders needs to do next is win over nonwhite voters

Yet Sanders still hasn't answered the biggest question of his campaign: Can he broaden his appeal beyond white Democrats?

As Dylan Matthews has written, the populations of Iowa and New Hampshire are both more than 87 percent white — and the United States as a whole is just 62 percent white. Furthermore, nonwhite voters make up an even larger share of the Democratic coalition, since so many more of them lean Democrat than Republican.

And Clinton has been clobbering Sanders among nonwhite Democrats. In a recent NBC News/SurveyMonkey national poll, Sanders beat Clinton by 2 points among white Democrats, but trailed her by 43 points among black Democrats and by 34 points among Hispanics.

The question, then, is whether these numbers will change. Shortly before the Iowa caucuses, I spoke to Rep. Keith Ellison (D-MN), an African American member of Congress who has endorsed Sanders, and he made the case that the senator's class-focused message will indeed appeal to black and Hispanic voters.

"Think about it," Ellison said. "Raising the minimum wage to $15 an hour is going to help mostly women, people of color, and low-income people. When you look at who’s on student debt, clearly a lot of white students have it, but proportionately a lot of black and brown voters have more." He continued: "The truth is that in America we have race and we have class, but they’re separate things that overlap a lot."

Sanders's main problem, Ellison believed, was just that he needed more "exposure." He went on: "The black community and brown community have known Hillary Clinton since the ‘90s. So she has quite an advantage in this area. Her husband was fairly popular among African Americans and other people, so she has a built-in advantage. I think Bernie is going to be able to solve his exposure problem by continuing to do the things that he’s doing."

The Vermont senator's tie in Iowa and win in New Hampshire will ensure he'll get a tremendous amount of that exposure in the coming days and weeks. His real first tests among nonwhite Democrats will be the Nevada caucuses, on February 20, and the South Carolina primary, on February 27. And if he manages to do well in those, it will be clear that Hillary Clinton's campaign is in very deep trouble.