so what exactly is his goal here? If he wants to encourage the Democratic Party to choose a lefty as their nominee, it would probably be better for him to run third party.

If he runs as a Democrat, he'll be nothing but a foil for Hillary. She'll shoot him down and gain points as a moderate, which is just what she would like. But I don't think he's running to be Hillary's foil in the debates.

If he really wants to motivate the Democrats to pick a progressive, I think it would be better for him to run third party. If the Democrats choose a third-way candidate, progressives might just defect and vote for Bernie in the general election. If they want to hold on to the progressives, they'll want to choose a lefty. Bernie might bow out of the race if the Democrats choose a progressive.

The question is, will the Democrats sacrifice the progressive vote to gain the ever-elusive center? I suppose it depends on the numbers. How many votes would they lose versus how many would they gain? Bernie could be an important influence on the election if he runs as an independent. If he runs as a Democrat, I think he'd probably help Hillary instead of moving the party left. I guess it all depends on what he really wants to accomplish by running.