Three weeks ago, a slightly disheveled U.S. senator wandered out to the front lawn of the Capitol Building and declared his campaign for president. Bernie Sanders, an avowed democratic socialist from the small state of Vermont, was in.

Some people aren't taking Bernie Sanders' campaign seriously, but that's a mistake.

The simplicity and directness of the setting signaled that Sanders wasn’t just another candidate in a crowded race, but a whole different kind of challenger. As Sanders was quick to warn, "People should not underestimate me.” He’s not kidding. As Sanders’ career has demonstrated, he doesn’t just hold policy positions, he acts on them.

1. He’s an Open Socialist

That Sanders is in the Senate at all is somewhat amazing. Sanders is the first Socialist elected in Senate history, no mean feat in the days when neoliberal economic policy rules.

But where it would have been the easiest thing in the world to change the (I) after his name to a (D), Sanders stuck with the affiliation. "I wouldn't deny it,” he said. “Not for one second. I'm a democratic socialist.” (note the “democratic” part; Sanders is quick to distance himself from the autocratic iterations of his philosophy.)

Not only does he not deny it, he flaunts it. New York Times political profilist Mark Leibovich paid the then just-elected socialist a visit in his brand new office and noticed the socialist accents were a bit heavy:

But he does little to airbrush the red “S” from his political profile. On the wall of his Congressional office hangs a portrait of Eugene V. Debs, the Socialist Party presidential candidate of the early 20th century. A poster in a conference room marks Burlington’s sister-city relationship with Puerto Cabeza, Nicaragua—one of a few such alliances he forged with cities in Marxist states during his 10-year stint as mayor of Vermont’s biggest city in the 1980s.

And where Reagan-inflected patriotism is the standard campaign rhetoric, Sanders openly praises Scandinavian socialism. “What's wrong with that?" he asked ABC News’ George Stephanopoulos. "What's wrong when you have more income and wealth equality? What's wrong when they have a stronger middle class in many ways than we do, higher minimum wage than we do and they are stronger on the environment than we do?"

2. He Trounced a Self-Funded Billionaire

Don’t knock winning as a socialist, not even in Vermont, which despite its solid-blue reputation voted for a Democratic president only once before 1992. (That would LBJ; the liberal northeastern Republicans balked at Barry Goldwater in 1964.)

But Sanders didn’t just win; he trounced a self-funded tech billionaire, the exact kind of person he warns is taking over the American political system. A novelist could not have devised a more fitting villain for Sanders’ 2006 campaign than business mogul Rich Tarrant. (Sanders, by contrast, is one of the few non-millionaires wandering the halls of the Senate.) Tarrant spent a whopping $7 million of his own money to defeat Sanders, turning the race into one of the most expensive Senate campaigns in history. He had little to show for it. Sanders beat him by a 2-1 margin, meaning Tarrant spent almost $100 per vote.

Along the way, Tarrant cut some vicious ads going after Sanders for out-of-context votes. Sanders’ distaste for negative campaigning lasts until today, as he reminds every cable news pundit who tries to make him take a potshot at Hillary Clinton.

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3. He Filibustered the Extension of the Bush Tax Cuts

In 2010, as the Senate considered the extension of the Bush tax cuts in a complicated lame-duck legislative maneuver, Sanders stood against extending the tax breaks for people making over $250,000—literally stood, for 8.5 hours, as he patiently explained the injustice of giving tax breaks to the wealthiest as the middle class struggled and wages stagnated. The tax package passed anyway, but Sanders helped set the opposition to the upper echelon of the Bush tax cuts that were finally nixed in 2013.

Sanders’ filibuster was notable for another reason; his marathon speech came in the midst of the Senate GOP’s use of the cloture filibuster, requiring 60 votes to bring a bill to discussion, which effectively ground the Senate to a halt for much of Obama’s presidency. The use of “filibusters” ballooned during this period, accounting for more filibusters than in the decades preceding it combined. But until then not one Republican actually stood up and did a real one. Sanders did.

4. He Caused Republicans to Run in Fear over a Climate Change Amendment

Tired of Republican politicians' “I’m not a scientist” escape hatch in response to questions about the existence of climate change, Sanders devised a way to force senators to say yes or no. He proposed an amendment to the Keystone Pipeline legislation. Sanders’ “sense of Congress” resolution would have forced senators to vote on whether:

“One, climate change is real. Two, climate change is caused by human activity. Three, climate change has already caused devastating problems in the United States and around the world. And, four, it is imperative the United States transform its energy system away from fossil fuels and toward energy efficiency and sustainable energy.”

The amendment so scared the Senate GOP that it managed to squelch the thing, even after Senate Majority Leader Mitch McConnell (R-KY) had said he was open to any and all amendments on the pipeline bill.

Sanders has been active on climate change in the Senate, but the chamber has not jibed with his sense of urgency. In 2013 Sanders and Senator Barbara Boxer (D-CA) introduced what they called a “gold standard” climate change bill, taxing carbon emissions to pay for a massive expansion in green energy industries while ending loopholes for fracking. The bill didn’t “settle for half-measures, but rather [laid] out an actual solution to the climate crisis.” Too ambitious for Congress, however, the bill died, and the Senate’s unwillingness to confront climate change only worsened with the 114th Congress that knocked the Democrats out of the majority.

Nonetheless, Sanders’ record on climate change looks to be one of the major selling points of his campaign. If he won’t win the nomination, he can influence the debate, in part by framing climate change as a primary issue. Sanders once co-authored a bill with then-Senator Hillary Clinton on stimulating green job creation, and as Clinton’s main liberal rival he’d be perfectly placed to press her on the issue at a time when the frontrunner is backing off of controversial policy positions.

5. He’s Keeping Hillary Clinton Honest on Trade

That’s not the only way a Sanders campaign could impact the Democratic primary. Sanders is liberal enough—and blunt enough—to keep Clinton honest on a range of issues, most recently the Trans-Pacific Partnership, a trade deal pitting progressives and organized labor against the centrist Democratic wing, including President Barack Obama.