In primary season, elections in this or that state often are called "pivotal." Far fewer really are. We won't know for sure (you never do) for a while, but in the Democratic race for president Michigan may have been an actual pivot.
There is of course the point that, had the polling been correct and Hillary Clinton won Michigan decisively (as she overwhelmingly won Mississippi the same day), Sanders would have been on the ropes. Even as matters stand, he's presently far behind - by about 200 - in the delegate count. And he won Michigan only by a modest margin.
But, well, polling was not correct, and to a degree that will go down in political lore. Harry Enten at the FiveThirtyEight site reflected, "Bernie Sanders made folks like me eat a stack of humble pie on Tuesday night. He won the Michigan primary over Hillary Clinton, 50 percent to 48 percent, when not a single poll taken over the last month had Clinton leading by less than 5 percentage points. In fact, many had her lead at 20 percentage points or higher. Sanders’s win in Michigan was one of the greatest upsets in modern political history."
That means, as people cast their ballots, they may pay a little less attention to the polling and to who's ahead. (And yes, those expectation factors really do drive votes.)
The Democratic race had been getting less attention in the last couple of weeks than the Republican, which hurts Sanders. The Michigan result provides a compelling argument for increasing attention given to the Democrats.
And, while Mississippi is a lot like many of the southern states which have been voting (strongly) for Clinton, those states are all gone now - all voted. The big votes ahead, in states like Ohio, Illinois, New York, California, will be much more like Michigan than like those southern states. Is something going on there that Sanders may be tapping into?
None of this is a prediction for a Sanders win. But the presidential abruptly looks different than it did yesterday.