Bernie Sanders offered some of his harshest criticisms of Hillary Clinton yet in a tweetstorm Wednesday, arguing that her September self-identification as a "moderate" is both accurate and incompatible with the idea that she is a progressive.

You can be a moderate. You can be a progressive. But you cannot be a moderate and a progressive. — Bernie Sanders (@BernieSanders) February 3, 2016

Sanders followed up that basic observation with a series of specific, issue-focused criticisms. He hit her on financial ties to Wall Street:

Most progressives that I know don't raise millions of dollars from Wall Street. — Bernie Sanders (@BernieSanders) February 3, 2016

And on trade:

Most progressives I know are firm from day 1 in opposition to the Trans-Pacific Partnership. They didn't have to think about it a whole lot. — Bernie Sanders (@BernieSanders) February 3, 2016

And on the environment:

Most progressives that I know were opposed to the Keystone pipeline from day one. Honestly, it wasn’t that complicated. — Bernie Sanders (@BernieSanders) February 3, 2016

And perhaps most notably on foreign policy, where Clinton has often been out of step with liberals but where Sanders has sometimes seemed a little uncomfortable discussing his own views:

Most progressives I know were against the war in Iraq. One of the worst foreign policy blunders in the history of the United States. — Bernie Sanders (@BernieSanders) February 3, 2016

These are all pretty standard from-the-left hits on Clinton. But the overall framing in terms of a progressive versus moderate dichotomy is also a reminder of exactly what makes a lot of Democratic leaders — including very progressive ones — nervous about Sanders.

There are enough self-identified conservatives in the United States that a Republican can win a presidential election while losing self-identified moderates. But a Democrat needs to win the votes of self-identified moderates by a healthy margin in order to have a chance, so Democrats who want to win need to bridge a divide between liberal and moderate self-conceptions.