Charles Pierce is on point here about the why and the how Sanders has made the primary race with Clinton a close fight:

The simple fact is that, if HRC has lost her lead at the moment, she has lost it to a superior campaign.

And it’s not as simple as the “populist anger” narrative would have you believe. Sanders has been running a 50-state campaign since before he formally declared his candidacy. He went to South Carolina. He went to Mississippi. He drew large and approving crowds in both places. He has stayed doggedly on message, directly refusing to help the elite political class in its pursuit of shiny objects. He repeatedly has emphasized that the pursuit of his policy goals, which all have to do with breaking the power of impending oligarchy and its threat to self-government, cannot be limited simply to electing him. And that’s where the easy narrative falls apart.

Pierce goes on to explain how Sanders’ populism is distinct from Trump, a meaningful difference in a media environment seeking easy answers for what the Beltway press finds to be two inexplicable campaigns:

Sanders punches up at the elites that, frankly, have more power in our politics than he does, or than you do, or than any politician does. He tells his audiences that he can’t do it alone, that the money power has grown too great for any one person to combat. He needs them more than they need him. He is not Napoleon, he is a democratic politician. And that makes all the difference and that’s why the “populist anger” narrative is a shuck. Anyone who says they could vote for either Bernie Sanders or He, Trump has been living for the last nine months with their head in a laundry bag.

The respective appeals of the two men are similar only on the simplest and least consequential levels. On the most profound levels, the two campaigns couldn’t be more different. Bernie Sanders is where he is because the positions and the policies he has been championing all his career have come back somewhat into favor ever since some grifters broke the world economy and then made off with the rubble. That is why he’s different from Donald Trump and that is why Hillary Rodham Clinton is noticing that things in the rear-view window are closer than they appear.