He's from Vermont, hardly a presidential power base. Sanders is a U.S. Senator who is officially an independent, although he describes himself as a Democratic Socialist, and even left-leaning Democrats don't embrace the S-word. And compared to Fortress Hillary, he has a tiny war chest and organization to wage a fight for the nomination.
Yet Bernie Sanders is making some noise. He finished a strong second to Mrs. Clinton in an admittedly unscientific Wisconsin straw poll, getting 41% to her 49%. She is still far ahead of the rest of the Democratic field in national polls, but finishing only eight points clear of Sanders in Wisconsin makes it look like maybe her candidacy isn't quite the cakewalk it's been made out to be.
This is not to suggest Sanders will win the nomination; he won't. But what this means is Hillary is going to face pressure from the left. She is going to have to pay attention to liberals who say she's too cozy with Wall Street, not attentive enough to the needs of working people, too eager to sign off on free trade agreements critics fear will allow China to siphon off American jobs.
The Hillary Clinton media storyline has provided her with an air of inevitability. Every little setback muddies her path to the White House and energizes Republicans. So while the nomination is certainly Hillary's to lose, she hasn't won it yet.