This is not, by any traditional definition, breaking news.  Sanders, aside from being an avowed Socialist, is little known outside of the most liberal of circles nationally and has no intention of matching Hillary Clinton dollar for fundraising dollar.  (Even if Sanders wanted to try to raise the sort of money that would make him competitive with Clinton, he couldn't do it. Or come anywhere close.)

No one knows those realities better than Sanders. So, why run? Because Sanders is part of a new-ish breed of presidential candidate: One who runs a cause more than campaign.

Sanders isn't, really, concerned with winning. He's been around politics long enough -- he's been in state and federal politics almost continuously since 1981 -- to understand how big a frontrunner Clinton is and to grasp his own limitations as a candidate. Of Clinton, he told the Post back in February: "Based on her history, do I think she is going to be as bold as needs to be in addressing the major crises that we face? Probably not."

What Sanders' candidacy is really about is influencing the debate within the Democratic party in the quadrennial pinchpoint of a presidential election. Sanders wants to drag Clinton (and everyone else in the field) to the left on issues like trade (he opposes the Trans-Pacific Partnership), campaign finance reform and income inequality.

It's a role he's quite familiar with.  Sanders rose to national prominence back in 2010 when he filibustered an extension of the Bush era tax cuts.

The Almanac of American Politics described Sanders' filibuster this way: "The speech proved so popular that it temporarily shut down the Senate video server and put his name atop Twitter's list of trending topics." (Twitter: So you know it mattered!)

The Bush tax cuts got extended -- but Sanders made his point, and his name.

That episode is a worthwhile blueprint for those seeking to understand what Sanders is up to (or will be up to) in the 2016 race. And, there's already some evidence that Sanders candidacy or, maybe more accurately, what his candidacy represents to the left, is having some impact.

Clinton has revealed very little of her 2016 policy platform but one place where she's seems likely to push hard is on the issue of campaign finance reform. In one of her first appearance as an official candidate, she told an Iowa audience that the campaign financing system was "dysfunctional" and in a (very brief) follow-up interview with two Post reporters she added: "We do have a plan. We have a plan for my plan." (Um, ok.)

She's also emphasized her bona fides on the income inequality fight in the early days of her campaign -- insisting that it's a fight that she's long been involved in and will take to Washington if elected next November. “Americans have fought their way back from tough economic times," she said in the video announcing her presidential candidacy. "But the deck is still stacked in favor of those at the top. Everyday Americans need a champion."

On trade -- and TPP, more specifically -- Clinton has avoided taking a position, notable because of President Obama's support for the deal. "Any trade deal has to produce jobs and raise wages and increase prosperity and protect our security,” Clinton said last week in New Hampshire, a statement that says a whole lot of not much.

That positioning is not because Clinton is nervous about the prospect of "Bernie for President." But, what Sanders' candidacy will do is ensure that someone in the race is holding Clinton's feet to the fire on these issues of import to the left. Sanders, in debates especially but on the campaign trail more generally, will push Clinton to not only affirm the general sentiments she's expressed on campaign finance, income inequality and trade but also to put out specifics of how exactly she would approach those issues if elected president.

Sure, everyone not named "Hillary Clinton" in the race will try to force Clinton to more clearly elucidate her stances on issues.  But, unlike, say, Martin O'Malley, who would love to wind up in the Clinton Cabinet, Sanders has nothing to lose. He's not going to be on the Democratic ticket or in Clinton's Cabinet no matter what happens. So, he's free to take the fight to Clinton in ways that others in the field simply won't.

That's not a strategy to win a presidential nomination. But, as a strategy to make a point, it's pretty damn good.

Sen. Bernie Sanders (I-Vt.), a White House contender in 2016, is known for his stances on budget issues and war. Here are his takes on Obamacare, Social Security and more. (Julie Percha/The Washington Post)