There's a lot of spin in Hillary Clinton campaign manager Robby Mook's Medium post "To Hillary Clinton supporters: The facts on where the race stands." Such as: "Hillary Clinton is the only candidate tough enough to beat Donald Trump." Um, okay?

But there's a paragraph in Mook's missive that is critically important -- particularly given that Bernie Sanders may well win the Wisconsin primary tonight, making it seven wins for the Vermont socialist in the last eight contests. Here it is:

Hillary Clinton has a lead of nearly 230 pledged delegates  —  and with each passing week, it’s becoming increasingly unlikely that Senator Sanders will be able to catch up. In order to do so, Sanders has to win the four remaining delegate-rich primaries —  New York, Pennsylvania, California, and New Jersey  —  with roughly 60 percent of the vote. To put that in perspective: Sanders has thus far won only two primaries with that margin: Vermont and New Hampshire. Needless to say, the size and demographic makeups of New York, Pennsylvania, California, and New Jersey are decidedly different than Vermont and New Hampshire. And these figures don’t even include superdelegates, where Clinton has an overwhelming lead.

Here's that paragraph visualized:

That Clinton lead may not look all that daunting until you remember this: There are no winner-take-all primaries or caucuses on the Democratic side. Every contest allocates delegates proportionally. What that means in simple terms is that it's hard for a front-runner to pull away and very hard for an underdog to close the gap.

Take Wisconsin, for example. Polling suggests Sanders is a narrow favorite -- with the RealClearPolitics polling average giving him a 2.6 percentage point edge. Let's say he wins by that margin. He would likely emerge with a tiny net delegate win, barely making a dent in Clinton's overall margin. This tweet from ABC's Ryan Struyk makes that point vividly:

Here's another way to think about Wisconsin vis-a-vis the New York primary in two weeks time, courtesy of our own Philip Bump:

To overcome Clinton's pledged delegate lead, Sanders has to win big states BIG. As Mook notes, Sanders needs to win 60 percent (or higher) of the vote in New York, Pennsylvania, New Jersey and California. Possible? Sure. Likely? Not even close -- especially when you consider (as Mook does) that the only two states where Sanders has achieved that margin are his home state and one that directly borders it.

None of the above means that Sanders can't or shouldn't stay in the Democratic primary race. He can and will, both to ensure that his liberal views (and those of his supporters) get a hearing and to be the fall-back plan in the event that the Justice Department finds Clinton criminally responsible for her decision to exclusively use a private email server while secretary of state.

What the math above does mean, however, is that barring some sort of cataclysm related to Clinton, Sanders simply isn't catching her in pledged delegates. Just. Not. Happening.