Bernie Sanders likes to think of himself as a different (and better) kind of politician. One of the manifestations of that image is that he has largely avoided any negative, personal attacks on Hillary Clinton during the Democratic presidential primary (no matter what the Clinton campaign claims).

No longer. Or, at least, not on Wednesday, when Sanders unleashed a broad-scale indictment of Clinton's qualifications to be president that touched on her closeness to major donors and corporations as well her judgment on the biggest foreign policy decision of the past two decades.

"She has been saying lately that she thinks I am quote-unquote 'not qualified to be president,' " Sanders said at a rally in Philadelphia. "Let me just say in response to Secretary Clinton, I don't believe that she is qualified if she is, through her super PAC, taking tens of millions of dollars in special-interest money. I don't think you are qualified if you get $15 million through Wall Street for your super PAC. ... I don't think you are qualified if you have voted for the disastrous war in Iraq."

This isn't exactly hand size and "Lyin' Ted" but, for Sanders and the Democratic race, it amounts to a genuine negative turn. Making a direct link between Clinton's donors and the purported access they are buying, arguing she lacks the proper judgment on foreign policy and insisting her trade views are wrongheaded is a step -- or several steps -- beyond where Sanders had been willing to go in the race up to this point.

Which raises two questions: 1) Why? and 2) Why now?

The answer to both is the same: For Sanders to have any plausible case to be the party's nominee, he needs to disrupt the race in a fundamental way.

But, he has won seven of the past eight votes, you say. He has outraised Clinton in each of the three months. How much more disruptive can he get?

All true! And also all not a fundamental disruption of the race. Sanders still trails Clinton by more than 200 pledged delegates. The math is close to determinative -- and not in his favor. Barring a cataclysm in the race, Clinton will be the nominee.

And the only potential cataclysm on the calendar anytime soon is the New York primary on April 19.

Sanders and his team understand that if he beats Clinton in New York -- a state she represented for eight years in the Senate and now calls home -- it would force a reexamination of the race from the party establishment, the media and the donor class. Although it wouldn't alter the delegate math (it's hard to see anything doing that), it would raise fundamental questions about whether Clinton can excite enough Democrats to win the White House in the fall. If Sanders won New York, the Clinton fretting industry, which has been dormant of late even amid her series of losses, would roar back with worries about what her inability to rally the party means.

New York then looks like Sanders' best, last chance to fundamentally shift the Clinton campaign car out of cruise control. And when you are down to your last chance, you do what needs to be done to win. For Sanders, that means going after Clinton's vulnerabilities more directly than he has done in the race to date.

One thing Sanders has yet to do: bring up Clinton's private email server and the ongoing Justice Department investigation of the propriety of that setup. In fact, when he has been asked about it, he has been entirely dismissive of it as a campaign issue. If Sanders changes his tune on that front in the next 12 days, then you know he's going for broke in New York.